Handle Google token failures and report Google Calendar status

When Google refuses the stored grant (invalid_grant on refresh, or HTTP
401 from the Calendar API to the access token just refreshed) on any
Calendar call - listing, choosing or pushing - the connection becomes
reconnect required under the account lock: the refresh token is cleared
and last_sync_error set to "Google access was revoked or expired —
reconnect". The call answers 409 google_reconnect_required, and later
calls answer the same without contacting Google until a reconnect or a
disconnect. The chosen calendar and stored calendar event ids are kept:
the pushed events are still on that calendar, so forgetEvents is not
run.

HTTP 429 and 5xx from the token endpoint or the Calendar API are retried
at most twice with backoff, then answered 503 google_unavailable,
retryable, without clearing or disconnecting anything.

Add GET /api/v1/integrations/google, reporting not_connected, connected
or reconnect_required with the chosen calendar and last_sync_error.

Migration 0015 rebuilds integrations_google so refresh_token_ct can be
cleared, with a CHECK that a row without a token carries the reason.

Maestro task b8d8da5a-0450-4571-963e-b3dfead24283.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/google.go

@@ -147,9 +147,8 @@ func putGoogleCalendar(logger *slog.Logger, svc *gcal.Service) http.HandlerFunc
 
 // writeGoogleCalendarError maps a calendar listing or choice error to its
 // API error: 409 not_connected when nothing is connected, 422
-// unknown_calendar for a calendar MealQuest cannot push to, 502
-// reconnect_required when Google no longer accepts the stored grant, and 502
-// google_failed when Google refused or could not be reached.
+// unknown_calendar for a calendar MealQuest cannot push to, and a Google
+// failure as writeGoogleFailure does.
 func writeGoogleCalendarError(w http.ResponseWriter, r *http.Request, logger *slog.Logger, action string, err error) {
 	var calendar *gcal.CalendarError
 	switch {
@@ -158,16 +157,47 @@ func writeGoogleCalendarError(w http.ResponseWriter, r *http.Request, logger *sl
 	case errors.Is(err, gcal.ErrUnknownCalendar):
 		writeError(w, r, logger, http.StatusUnprocessableEntity, "unknown_calendar",
 			"choose one of your calendars that MealQuest can add dinners to")
-	case errors.Is(err, gcal.ErrReconnect):
-		logger.WarnContext(r.Context(), action, "error", err)
-		writeError(w, r, logger, http.StatusBadGateway, "reconnect_required",
-			"Google no longer accepts MealQuest's access to your calendar; connect Google Calendar again")
-	case errors.As(err, &calendar):
-		logger.WarnContext(r.Context(), action, "error", err)
-		writeError(w, r, logger, http.StatusBadGateway, "google_failed",
-			"Google Calendar could not be reached; try again")
+	case errors.Is(err, gcal.ErrReconnect), errors.As(err, &calendar):
+		writeGoogleFailure(w, r, logger, action, err, "Google Calendar could not be reached; try again")
 	default:
 		logger.ErrorContext(r.Context(), action, "error", err)
 		writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
 	}
 }
+
+// writeGoogleFailure answers err, gcal.ErrReconnect or a *gcal.CalendarError,
+// the way every Google Calendar call does: 409 google_reconnect_required when
+// Google refused the stored grant (recorded as reconnect required, so only
+// connecting again helps), 503 google_unavailable, retryable, when Google
+// answered every try with HTTP 429 or 5xx (nothing was disconnected), and
+// 502 google_failed, with failed as its message, for any other failure.
+func writeGoogleFailure(w http.ResponseWriter, r *http.Request, logger *slog.Logger, action string, err error, failed string) {
+	logger.WarnContext(r.Context(), action, "error", err)
+	var calendar *gcal.CalendarError
+	switch {
+	case errors.Is(err, gcal.ErrReconnect):
+		writeError(w, r, logger, http.StatusConflict, "google_reconnect_required",
+			"Google access was revoked or expired; connect Google Calendar again")
+	case errors.As(err, &calendar) && calendar.Retryable():
+		writeAPIError(w, r, logger, http.StatusServiceUnavailable, apiError{Code: "google_unavailable",
+			Message: "Google Calendar is busy right now; try again in a few minutes", Retryable: true})
+	default:
+		writeError(w, r, logger, http.StatusBadGateway, "google_failed", failed)
+	}
+}
+
+// getGoogle answers the Google Calendar connection's status: {status,
+// calendar_id, calendar_name, connected_at, last_sync_error}, where status
+// is not_connected, connected or reconnect_required.
+func getGoogle(logger *slog.Logger, svc *gcal.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Cache-Control", "no-store")
+		st, err := svc.Status(r.Context())
+		if err != nil {
+			logger.ErrorContext(r.Context(), "read Google Calendar status", "error", err)
+			writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+			return
+		}
+		writeJSON(w, r, logger, http.StatusOK, st)
+	}
+}