refactor(medications): drop the legacy today walker and its own zone solver

expandTodayIntakes had no caller left once the today projection moved onto
the recurrence engine and localHmAsUtc; it carried the last private
wall-clock solver in the medication code. The recurrence engine now takes
its offset and UTC-day helpers from the tz module instead of local copies.

src/lib/__tests__/medication-schedule.test.ts

@@ -1,9 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
-  expandTodayIntakes,
   parseScheduleRecurrence,
   serializeScheduleRecurrence,
-  type TodayExpandableSchedule,
 } from "@/lib/medication-schedule";
 
 describe("medication schedule recurrence", () => {
@@ -54,111 +52,3 @@ describe("medication schedule recurrence", () => {
     ).toBe("i4;");
   });
 });
-
-describe("expandTodayIntakes", () => {
-  // 2026-05-21 is a Thursday → weekday 4. Pin a reference instant
-  // safely inside the Europe/Berlin day so every test below shares
-  // the same "today" anchor regardless of the host clock.
-  const REFERENCE = new Date("2026-05-21T12:00:00Z");
-  const TZ = "Europe/Berlin";
-
-  const daily: TodayExpandableSchedule = {
-    id: "sched-daily",
-    medicationId: "med-daily",
-    windowStart: "07:00",
-    windowEnd: "09:00",
-    daysOfWeek: null,
-  };
-
-  it("emits a slot for a schedule with daysOfWeek=null (every day)", () => {
-    // This is the bug the W-SERVER-FIX patch addresses: the operator's
-    // Ramipril (Morgens) + Metformin (Abends) both ship `daysOfWeek:
-    // null` in the DB ("every day" per the schema annotation) and the
-    // today-intake endpoint returned `[]`. With the fix, the projector
-    // emits one slot per schedule.
-    const slots = expandTodayIntakes([daily], REFERENCE, TZ);
-    expect(slots).toHaveLength(1);
-    expect(slots[0]).toMatchObject({
-      scheduleId: "sched-daily",
-      medicationId: "med-daily",
-    });
-  });
-
-  it('treats daysOfWeek="" (empty string) as every day', () => {
-    const slots = expandTodayIntakes(
-      [{ ...daily, daysOfWeek: "" }],
-      REFERENCE,
-      TZ,
-    );
-    expect(slots).toHaveLength(1);
-  });
-
-  it("treats all-seven-days (0..6) as every day", () => {
-    const slots = expandTodayIntakes(
-      [{ ...daily, daysOfWeek: "0,1,2,3,4,5,6" }],
-      REFERENCE,
-      TZ,
-    );
-    expect(slots).toHaveLength(1);
-  });
-
-  it("respects an explicit weekday filter that excludes today", () => {
-    // Thursday is weekday 4; "1" = Monday only → no slot today.
-    const slots = expandTodayIntakes(
-      [{ ...daily, daysOfWeek: "1" }],
-      REFERENCE,
-      TZ,
-    );
-    expect(slots).toHaveLength(0);
-  });
-
-  it("includes a slot when the explicit weekday filter contains today", () => {
-    // Thursday = weekday 4; allow {2,4} → today matches.
-    const slots = expandTodayIntakes(
-      [{ ...daily, daysOfWeek: "2,4" }],
-      REFERENCE,
-      TZ,
-    );
-    expect(slots).toHaveLength(1);
-  });
-
-  it("emits slots for two daysOfWeek-null schedules (Ramipril + Metformin shape)", () => {
-    // The operator's real fixture: morning + evening daily meds.
-    const morning: TodayExpandableSchedule = {
-      id: "sched-ramipril",
-      medicationId: "med-ramipril",
-      windowStart: "07:00",
-      windowEnd: "09:00",
-      daysOfWeek: null,
-    };
-    const evening: TodayExpandableSchedule = {
-      id: "sched-metformin",
-      medicationId: "med-metformin",
-      windowStart: "19:00",
-      windowEnd: "21:00",
-      daysOfWeek: null,
-    };
-    const slots = expandTodayIntakes([morning, evening], REFERENCE, TZ);
-    expect(slots).toHaveLength(2);
-    expect(slots.map((s) => s.medicationId).sort()).toEqual([
-      "med-metformin",
-      "med-ramipril",
-    ]);
-  });
-
-  it("anchors scheduledFor at the schedule's windowStart in the user tz", () => {
-    const slots = expandTodayIntakes([daily], REFERENCE, TZ);
-    expect(slots).toHaveLength(1);
-    const iso = slots[0].scheduledFor.toISOString();
-    // 07:00 Europe/Berlin on 2026-05-21 (DST in effect → UTC+2) → 05:00Z.
-    expect(iso).toBe("2026-05-21T05:00:00.000Z");
-  });
-
-  it("skips multi-week cadence (handled by the reminder worker)", () => {
-    const biweekly: TodayExpandableSchedule = {
-      ...daily,
-      daysOfWeek: "i2;1,3,5",
-    };
-    expect(expandTodayIntakes([biweekly], REFERENCE, TZ)).toEqual([]);
-  });
-});

src/lib/medication-schedule.ts

@@ -3,41 +3,6 @@ export interface ScheduleRecurrence {
   intervalWeeks: number;
 }
 
-/**
- * Minimal schedule shape consumed by `expandTodayIntakes`. Mirrors the
- * relevant subset of the Prisma `MedicationSchedule` row (`id`,
- * `windowStart`, `windowEnd`, `daysOfWeek`) plus the parent
- * `medicationId` so the projected slots carry enough context for the
- * intake-route caller to upsert into `MedicationIntakeEvent` without
- * re-joining.
- *
- * `daysOfWeek` is the encoded string per `serializeScheduleRecurrence`
- * (e.g. `null` for daily, `"1,3,5"` for Mon/Wed/Fri, `"i2;1"` for every
- * other Monday). The DB schema column documents `null = daily` as the
- * convention every reader honours.
- */
-export interface TodayExpandableSchedule {
-  id: string;
-  medicationId: string;
-  /** "HH:mm" 24h, user-tz reference. */
-  windowStart: string;
-  /** "HH:mm" 24h. May wrap midnight (`windowEnd < windowStart`). */
-  windowEnd: string;
-  /** Encoded recurrence string per `serializeScheduleRecurrence`. */
-  daysOfWeek: string | null;
-}
-
-/**
- * One projected dose slot for "today" in the user's timezone. The
- * `scheduledFor` is the UTC instant that corresponds to the schedule's
- * `windowStart` applied to today's local-day boundary in `timeZone`.
- */
-export interface TodayExpandedSlot {
-  scheduleId: string;
-  medicationId: string;
-  scheduledFor: Date;
-}
-
 const ENCODED_PATTERN = /^i([1-4]);(.*)$/;
 
 function normalizeDays(days: number[]): number[] {
@@ -90,147 +55,3 @@ export function serializeScheduleRecurrence(
   if (intervalWeeks === 1) return days.join(",");
   return `i${intervalWeeks};${days.join(",")}`;
 }
-
-/**
- * Project the supplied schedules to the dose slots they would produce
- * for "today" in the user's timezone. Treats `daysOfWeek` of `null`,
- * `""`, or "no valid weekdays after normalization" as "every day" — the
- * documented DB convention (`prisma/schema.prisma` annotates the column
- * `null = daily`). Explicit weekday arrays (`"1,3,5"`) still filter as
- * before, so a Monday-only schedule emits nothing on a Sunday.
- *
- * Used by `/api/medications/intake?scope=today` to backfill placeholder
- * `MedicationIntakeEvent` rows for active medications whose reminder
- * window has not yet opened — without this projection the endpoint
- * returned `[]` for daily meds before the reminder worker entered the
- * RED phase, leaving the iOS Dashboard tile + the "Erfassen" sheet
- * empty for the whole morning.
- *
- * The function is pure (no DB access) so the test surface stays inside
- * this file. The route owns the upsert.
- */
-export function expandTodayIntakes(
-  schedules: TodayExpandableSchedule[],
-  reference: Date,
-  timeZone: string,
-): TodayExpandedSlot[] {
-  const slots: TodayExpandedSlot[] = [];
-  const today = localDayParts(reference, timeZone);
-
-  for (const schedule of schedules) {
-    const recurrence = parseScheduleRecurrence(schedule.daysOfWeek);
-
-    // Day-of-week filter. Empty array (the parsed shape for `null` /
-    // `""` / "no valid weekdays") means "every day" — no filter.
-    if (
-      recurrence.daysOfWeek.length > 0 &&
-      !recurrence.daysOfWeek.includes(today.weekday)
-    ) {
-      continue;
-    }
-
-    // Multi-week cadence isn't anchored without a medication start
-    // date here, so `intervalWeeks > 1` is conservatively skipped from
-    // the today projection. The reminder worker still mints those rows
-    // when their window opens because it has the medication context;
-    // those meds are GLP-1-style weeklies which the iOS today list
-    // doesn't depend on as a primary surface.
-    if (recurrence.intervalWeeks > 1) continue;
-
-    const [h, m] = schedule.windowStart.split(":").map(Number);
-    if (!Number.isFinite(h) || !Number.isFinite(m)) continue;
-
-    const scheduledFor = instantAtLocalHm(today, h, m, timeZone);
-    slots.push({
-      scheduleId: schedule.id,
-      medicationId: schedule.medicationId,
-      scheduledFor,
-    });
-  }
-
-  return slots;
-}
-
-interface LocalDayParts {
-  year: number;
-  month: number;
-  day: number;
-  weekday: number;
-}
-
-const WEEKDAY_MAP: Record<string, number> = {
-  Sun: 0,
-  Mon: 1,
-  Tue: 2,
-  Wed: 3,
-  Thu: 4,
-  Fri: 5,
-  Sat: 6,
-};
-
-function localDayParts(date: Date, timeZone: string): LocalDayParts {
-  const parts = new Intl.DateTimeFormat("en-US", {
-    timeZone,
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-    weekday: "short",
-  }).formatToParts(date);
-  const get = (type: Intl.DateTimeFormatPartTypes): string =>
-    parts.find((p) => p.type === type)?.value ?? "0";
-  return {
-    year: Number(get("year")),
-    month: Number(get("month")),
-    day: Number(get("day")),
-    weekday: WEEKDAY_MAP[get("weekday")] ?? 0,
-  };
-}
-
-function instantAtLocalHm(
-  day: LocalDayParts,
-  hour: number,
-  minute: number,
-  timeZone: string,
-): Date {
-  // Two-pass solver — first guess assumes wall-clock equals UTC, then
-  // correct by the zone's offset at that approximate instant. Two
-  // iterations suffice for every IANA zone (the second pass adjusts
-  // across DST transitions).
-  let guess = new Date(
-    Date.UTC(day.year, day.month - 1, day.day, hour, minute, 0, 0),
-  );
-  for (let i = 0; i < 2; i++) {
-    const offsetMin = tzOffsetMinutes(guess, timeZone);
-    guess = new Date(
-      Date.UTC(day.year, day.month - 1, day.day, hour, minute, 0, 0) -
-        offsetMin * 60_000,
-    );
-  }
-  return guess;
-}
-
-function tzOffsetMinutes(date: Date, timeZone: string): number {
-  const parts = new Intl.DateTimeFormat("en-US", {
-    timeZone,
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-    hour: "2-digit",
-    minute: "2-digit",
-    second: "2-digit",
-    hour12: false,
-  }).formatToParts(date);
-  const get = (type: Intl.DateTimeFormatPartTypes): number =>
-    Number(parts.find((p) => p.type === type)?.value ?? "0");
-  let hour = get("hour");
-  if (hour === 24) hour = 0;
-  const asIfUtc = Date.UTC(
-    get("year"),
-    get("month") - 1,
-    get("day"),
-    hour,
-    get("minute"),
-    get("second"),
-  );
-  return Math.round((asIfUtc - date.getTime()) / 60000);
-}

src/lib/medications/scheduling/recurrence.ts

@@ -4,10 +4,9 @@
  * Single source of truth for "what dose slots does this schedule emit
  * between A and B?" The canonical engine is introduced in this release;
  * the v1.5.0 cut wires only the reminder worker (via
- * `worker-helpers.ts`) through it. The today-projector
- * (`expandTodayIntakes`), the cadence chart (`expandScheduleSlots`),
- * and the form-level helpers continue on the legacy walker through
- * v1.5.x and migrate per the read-flip plan. The medication card's
+ * `worker-helpers.ts`) through it. The today-projector, the cadence
+ * chart (`expandScheduleSlots`), and the form-level helpers continue on
+ * the legacy walker through v1.5.x and migrate per the read-flip plan. The medication card's
  * "next intake" line reads the server-computed `nextDueAt` (this engine
  * via `computeNextDueAt`) directly as of v1.8.4.
  *
@@ -30,7 +29,7 @@
  *      are NULL and `oneShot` is false. Decodes the legacy
  *      `daysOfWeek` string via `parseScheduleRecurrence` and emits
  *      weekly slots. **Honours `intervalWeeks > 1` correctly** —
- *      the existing `expandTodayIntakes` skipped it (the legacy
+ *      the retired today-projector walker skipped it (the legacy
  *      bi-weekly worker bug R-3 finding 5 calls out); this engine
  *      anchors the week phase to `startsOn ?? createdAt` and emits
  *      on the matching weeks only.
@@ -60,6 +59,8 @@ import { annotate } from "@/lib/logging/context";
 import { parseScheduleRecurrence } from "@/lib/medication-schedule";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import { startOfLocalDayInTz } from "@/lib/tz/local-day";
+import { tzOffsetMinutes } from "@/lib/tz/format";
+import { startOfUtcDay } from "@/lib/tz/start-of-utc-day";
 import { hhmmToMinutesOrNull } from "@/lib/medications/scheduling/hhmm";
 
 /**
@@ -906,8 +907,7 @@ function startOfCivilWeek(day: Date): Date {
 
 /**
  * Materialise an "HH:mm" on a civil day in the user's IANA timezone,
- * returning the corresponding UTC instant. DST-aware via the two-pass
- * solver pattern from `medication-schedule.ts`: a wall-clock time that
+ * returning the corresponding UTC instant. DST-aware: a wall-clock time that
  * does not exist on a spring-forward day resolves forward (02:30 in
  * Europe/Berlin on 2026-03-29 becomes 03:30).
  *
@@ -955,19 +955,6 @@ function applyTimeOfDayToCivilDay(day: Date, hhmm: string, tz: string): Date {
   return before;
 }
 
-function tzOffsetMinutes(date: Date, tz: string): number {
-  const parts = wallClockInTz(date, tz);
-  const asIfUtc = Date.UTC(
-    parts.year,
-    parts.month - 1,
-    parts.day,
-    parts.hour,
-    parts.minute,
-    parts.second,
-  );
-  return Math.round((asIfUtc - date.getTime()) / 60_000);
-}
-
 function buildOccurrence(
   at: Date,
   timeOfDay: string,
@@ -1011,12 +998,6 @@ function graceWindowMs(schedule: CanonicalSchedule): number {
 // UTC date helpers (RRULE day-anchor + endsOn cap arithmetic)
 // ────────────────────────────────────────────────────────────────────
 
-function startOfUtcDay(d: Date): Date {
-  return new Date(
-    Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), 0, 0, 0, 0),
-  );
-}
-
 function endOfUtcDay(d: Date): Date {
   return new Date(
     Date.UTC(