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/caldav_router.py

@@ -337,6 +337,7 @@ def get_events(
                 sync_errors.append({
                     "source": "caldav",
                     "name": f"{account.username} – {calendar.name}",
+                    "calendar_id": calendar.id,
                     "message": "Sync fehlgeschlagen",
                 })