feat(maintenance): DB maintenance UI card + fix ruff I001

- Settings → Maintenance gains a "Database maintenance" card: a "Run VACUUM
  ANALYZE now" button (enqueues the maintenance task) plus a per-table bloat
  readout (live/dead/dead%/last vacuum) from /api/admin/maintenance/db-stats.
- dbMaintenance store (loadStats / runVacuum) + test.
- Fix ruff I001: combine the two _sync_engine imports onto one line.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/tasks/maintenance.py

@@ -22,10 +22,7 @@ from ..models import (
     TaskRun,
 )
 from ..utils.phash import compute_phash
-from ._sync_engine import (
-    get_sync_engine,
-    sync_session_factory as _sync_session_factory,
-)
+from ._sync_engine import get_sync_engine, sync_session_factory as _sync_session_factory
 
 log = logging.getLogger(__name__)