feat(lint): refuse UTC day keys outside the time zone module

`instant.toISOString().slice(0, 10)` and parsing a key back as UTC
midnight are the idioms behind every day-boundary fix this year: the
evening west of UTC and the small hours east of it land on the
neighbouring day. A new rule, healthlog/no-utc-day-key, refuses them in
src/ outside src/lib/tz and tests, and refuses a disable directive for
itself that is not next-line or gives no reason.

The time zone module now names every legitimate form: userDayKey for a
person's day, dateOnlyKey / dateOnlyAtNoonUtc / dayKeyAsUtcMidnight for
date-only values and day labels, daysBetweenDateKeys / weekdayOfDateKey /
isCalendarDateKey / statedDateKey for key arithmetic and parsing.

Of the 133 sites the rule found, 84 are gone. Most moved onto a named
helper with no change in behaviour; these changed behaviour:

- cycle calendar, insights, Coach snapshot and prediction refresh start
  their lookbacks at the first instant of the user's day
- a model-transcribed date like "June 10, 2026" no longer lands a day
  early in a process east of UTC (documents and lab scans)
- a custom report range covers exactly the picked days in the profile
  zone instead of starting the evening before west of UTC
- date-of-birth pickers cap at the user's today, not the UTC one
- an allergy onset date is stored at noon UTC like other date-only values
- the report's cycle summary, the Coach's usual BP range and the intake
  projector's rollup recompute use the user's day
- vaccination dates render as dates instead of the previous evening

49 remain, each with its reason on the line above: 39 UTC by design (15
file name stamps, rollup and object keys, the UTC-day score engines) and
10 marked baseline. A guard test requires a classified reason on every
directive and holds the baseline to a ceiling that only goes down.

src/app/api/record-settings/[family]/route.ts

@@ -41,6 +41,7 @@ import {
   parseNotificationPrefs,
   resolveNotificationPrefs,
 } from "@/lib/validations/notification-prefs";
+import { dayKeyAsUtcMidnight, dateOnlyKey } from "@/lib/tz/date-only";
 
 export const dynamic = "force-dynamic";
 
@@ -93,7 +94,9 @@ export const GET = apiHandler(
         settings = {
           displayName: record.displayName,
           heightCm: record.heightCm,
-          dateOfBirth: record.dateOfBirth?.toISOString().slice(0, 10) ?? null,
+          dateOfBirth: record.dateOfBirth
+            ? dateOnlyKey(record.dateOfBirth)
+            : null,
           gender: record.gender,
           locale: record.locale,
           timezone: record.timezone,
@@ -195,7 +198,7 @@ export const PATCH = apiHandler(
             ...(patch.dateOfBirth !== undefined
               ? {
                   dateOfBirth: patch.dateOfBirth
-                    ? new Date(`${patch.dateOfBirth}T00:00:00.000Z`)
+                    ? dayKeyAsUtcMidnight(patch.dateOfBirth)
                     : null,
                 }
               : {}),
@@ -235,7 +238,9 @@ export const PATCH = apiHandler(
           }
           settings = {
             ...record,
-            dateOfBirth: record.dateOfBirth?.toISOString().slice(0, 10) ?? null,
+            dateOfBirth: record.dateOfBirth
+              ? dateOnlyKey(record.dateOfBirth)
+              : null,
           };
           changed = Object.keys(patch);
           break;