fix: keep closed week snapshot in sync when days change

When a day is closed, re-closed, or reopened, DayService now
recomputes worked_ms and delta_ms on the closed week containing
that day (if the week is already closed). This prevents stale
delta values after editing entries and re-closing a day.

- DayService.recomputeWeek: sums worked_ms from all closed_days
  in the week, updates closed_weeks row preserving expected_ms
- NewDayService now takes ClosedWeekStore
- WeekKeyForDayKey exported helper (used by DayService)
- TestWeekSnapshotUpdatesWhenDayReopened regression test

internal/service/week_service.go

@@ -45,6 +45,16 @@ func NewWeekService(
 // WeekDayKeysExported is exported for testing.
 var WeekDayKeysExported = weekDayKeys
 
+// WeekKeyForDayKey returns the ISO week key (YYYY-Www) for a given YYYY-MM-DD day key.
+func WeekKeyForDayKey(dayKey string, tz *time.Location) (string, error) {
+	t, err := time.ParseInLocation("2006-01-02", dayKey, tz)
+	if err != nil {
+		return "", fmt.Errorf("invalid day_key %q: %w", dayKey, err)
+	}
+	year, week := t.ISOWeek()
+	return fmt.Sprintf("%d-W%02d", year, week), nil
+}
+
 // weekDayKeys returns the YYYY-MM-DD keys for Mon-Sun of the ISO week encoded as weekKey.
 // weekKey format: "YYYY-Www" (e.g. "2024-W03").
 func weekDayKeys(weekKey string, tz *time.Location) ([]string, error) {