fix: include calendar_id in per-calendar sync errors

Events already carry calendar_id, but the corresponding sync-error
entries didn't, forcing clients to match errors to calendars by
fragile name-suffix comparison. Account-level failures (whole account
unreachable) still omit calendar_id since no single calendar is at fault.

backend/routers/google_router.py

@@ -418,6 +418,7 @@ def get_google_events(account: models.GoogleAccount, start_dt: datetime, end_dt:
             errors.append({
                 "source": "google",
                 "name": f"{account.email} – {gcal.name}",
+                "calendar_id": gcal.id,
                 "message": "Sync fehlgeschlagen",
             })