feat(profile): add date-format preference column

Mirror the time-format preference with a per-user date display
choice. AUTO follows the locale convention, DMY/MDY/YMD pin the
field order. Additive, rerun-safe migration; existing rows read AUTO.

prisma/migrations/0192_v1210_date_format/migration.sql

@@ -0,0 +1,24 @@
+-- v1.21.0 — per-user date display preference.
+--
+-- The UI locale alone decided whether dates rendered as dd.MM.yyyy or
+-- MM/dd/yyyy, and the native date input fell back to the browser locale,
+-- which left non-US self-hosters on MM/DD/YYYY with no way out. The new
+-- column carries an explicit preference: AUTO follows the locale
+-- convention, DMY pins day-month-year, MDY pins month-day-year, YMD pins
+-- ISO yyyy-MM-dd. Display-time only — stored instants stay UTC.
+--
+-- Additive + non-destructive: a new NOT NULL column with a default, no
+-- backfill needed. Existing rows read AUTO. Guarded so a re-run is a
+-- no-op (the enum create + the column add both skip when present).
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1 FROM pg_type WHERE typname = 'date_format_preference'
+  ) THEN
+    CREATE TYPE "date_format_preference" AS ENUM ('AUTO', 'DMY', 'MDY', 'YMD');
+  END IF;
+END
+$$;
+
+ALTER TABLE "users"
+  ADD COLUMN IF NOT EXISTS "date_format" "date_format_preference" NOT NULL DEFAULT 'AUTO';

prisma/schema.prisma

@@ -228,6 +228,11 @@ model User {
   // (en → AM/PM, de → 24h), H12 forces AM/PM, H24 forces 24-hour.
   // Display-time only — canonical storage stays UTC instants.
   timeFormat              TimeFormatPreference @default(AUTO) @map("time_format")
+  // Date display preference. AUTO follows the active locale convention
+  // (de → dd.MM.yyyy, en → MM/dd/yyyy); DMY pins day-month-year,
+  // MDY pins month-day-year, YMD pins ISO yyyy-MM-dd. Display-time only —
+  // canonical storage stays UTC instants.
+  dateFormat              DateFormatPreference @default(AUTO) @map("date_format")
   // AI provider selection for insights. Null = OPENAI fallback.
   aiProvider              String? @map("ai_provider") // "OPENAI" | "ANTHROPIC" | "LOCAL" | "CHATGPT_OAUTH"
   aiModel                 String? @map("ai_model")
@@ -973,6 +978,17 @@ enum TimeFormatPreference {
   @@map("time_format_preference")
 }
 
+// Date display preference for rendered dates. AUTO defers to the active
+// locale's convention; DMY/MDY/YMD pin the field order regardless of locale.
+enum DateFormatPreference {
+  AUTO
+  DMY
+  MDY
+  YMD
+
+  @@map("date_format_preference")
+}
+
 enum MeasurementSource {
   MANUAL
   WITHINGS