refactor(insights): dedup BERLIN_DAY_FORMATTER across seven status helpers

Each of the seven src/lib/insights/*-status.ts files carried an identical
20-LOC block defining BERLIN_DAY_FORMATTER plus toBerlinDayKey. Extract
the helper into src/lib/tz/resolver.ts (next to dayKeyForScheduledFor and
friends) and import it everywhere. Same runtime behaviour, 100-LOC drop.

src/lib/insights/blood-pressure-status.ts

@@ -30,29 +30,10 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/bmi-status.ts

@@ -15,29 +15,10 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/general-status.ts

@@ -20,13 +20,7 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
@@ -34,19 +28,6 @@ type SupportedLocale = "de" | "en";
 // in the AI general-status fetch (V3 audit: enum drift cousins).
 const MEASUREMENT_TYPES = measurementTypeEnum.options;
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/medication-compliance-status.ts

@@ -20,18 +20,12 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 // 360 daily days + 24 monthly windows ≈ 1080 days of intake history.
 const COMPLIANCE_HISTORY_DAYS = 360 + 24 * 30;
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
-
 type SupportedLocale = "de" | "en";
 
 interface MedicationSummaryItem {
@@ -39,19 +33,6 @@ interface MedicationSummaryItem {
   text: string;
 }
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/mood-status.ts

@@ -22,29 +22,10 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/pulse-status.ts

@@ -21,29 +21,10 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/insights/weight-status.ts

@@ -25,29 +25,10 @@ import {
 } from "@/lib/insights/with-timeout";
 import { persistTimeoutStubAndReturn } from "@/lib/insights/persist-timeout-stub";
 import { annotate } from "@/lib/logging/context";
-
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
+import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 type SupportedLocale = "de" | "en";
 
-function toBerlinDayKey(date: Date): string {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
 function round(value: number, digits = 1): number {
   const factor = 10 ** digits;
   return Math.round(value * factor) / factor;

src/lib/tz/resolver.ts

@@ -55,6 +55,38 @@ export {
   type FormatInUserTzShape,
 } from "./format";
 
+const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
+  timeZone: "Europe/Berlin",
+  year: "numeric",
+  month: "2-digit",
+  day: "2-digit",
+});
+
+/**
+ * Format a Date as `YYYY-MM-DD` in `Europe/Berlin`. The seven Insight
+ * status helpers (`bmi`, `blood-pressure`, `weight`, `pulse`, `mood`,
+ * `medication-compliance`, `general`) all key their per-day cache on
+ * this — Marc's primary tenant runs in Berlin and the cache action
+ * row reads identically across helpers when the formatter lives here.
+ *
+ * For per-user surfaces, prefer `userDayKey(date, tz)` from `./format`.
+ * This helper exists for the cache-key path where the day-bucket is
+ * intentionally Berlin-anchored regardless of the requesting user's
+ * stored zone.
+ */
+export function toBerlinDayKey(date: Date): string {
+  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
+  const year = parts.find((part) => part.type === "year")?.value;
+  const month = parts.find((part) => part.type === "month")?.value;
+  const day = parts.find((part) => part.type === "day")?.value;
+
+  if (!year || !month || !day) {
+    throw new Error("Could not derive Berlin day key");
+  }
+
+  return `${year}-${month}-${day}`;
+}
+
 const TTL_MS = 60_000;
 
 interface CacheEntry {