feat(maintenance): scheduled + manual DB VACUUM ANALYZE + bloat readout

The TABLESAMPLE showcase reads physical blocks (bloat-sensitive), and the
periodic prune/backfill/recovery tasks churn dead tuples faster than
autovacuum always keeps up — so explicit maintenance earns its keep here.

- tasks.maintenance.vacuum_analyze: VACUUM (ANALYZE) over high-churn tables
  (VACUUM_TABLES) on an AUTOCOMMIT connection (VACUUM can't run in a txn).
  Scheduled weekly via Beat; also operator-triggerable.
- _sync_engine.get_sync_engine(): expose the process engine for the
  autocommit connection.
- GET  /api/admin/maintenance/db-stats: per-table n_live/n_dead/dead_pct +
  last (auto)vacuum/analyze from pg_stat_user_tables — visibility, not a
  black box.
- POST /api/admin/maintenance/vacuum: enqueue the task on demand.

Tests: vacuum task runs + reports tables; db-stats shape; trigger queues.

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

backend/app/tasks/_sync_engine.py

@@ -34,3 +34,10 @@ def sync_session_factory():
         )
         _SESSIONMAKER = sessionmaker(_ENGINE, expire_on_commit=False)
     return _SESSIONMAKER
+
+
+def get_sync_engine():
+    """The process-wide sync Engine — for raw work that needs a connection
+    directly (e.g. AUTOCOMMIT VACUUM, which can't run inside a transaction)."""
+    sync_session_factory()  # ensure _ENGINE is initialized
+    return _ENGINE

backend/app/tasks/maintenance.py

@@ -22,10 +22,29 @@ from ..models import (
     TaskRun,
 )
 from ..utils.phash import compute_phash
-from ._sync_engine import 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__)
 
+# High-churn tables whose dead-tuple bloat matters: the TABLESAMPLE showcase
+# reads physical blocks (bloat slows it directly), and the periodic
+# prune/backfill/recovery tasks generate dead tuples faster than autovacuum
+# always keeps up with. VACUUM reclaims them; ANALYZE refreshes planner stats.
+# Allowlist ONLY — names are interpolated into VACUUM, so they must never come
+# from request input.
+VACUUM_TABLES = (
+    "image_record",
+    "image_provenance",
+    "post_attachment",
+    "download_event",
+    "task_run",
+    "import_task",
+    "import_batch",
+)
+
 STUCK_THRESHOLD_MINUTES = 5
 # Archive ImportTasks run the per-member pipeline inline for every
 # member (import_archive_file: soft=30min/hard=35min). The ImportTask
@@ -761,3 +780,20 @@ def cleanup_old_download_events() -> int:
         )
         session.commit()
         return result.rowcount or 0
+
+
+@celery.task(name="backend.app.tasks.maintenance.vacuum_analyze")
+def vacuum_analyze() -> dict:
+    """Periodic VACUUM (ANALYZE) over the high-churn tables (VACUUM_TABLES) to
+    reclaim dead-tuple bloat and refresh planner statistics. VACUUM cannot run
+    inside a transaction block, so it runs on an AUTOCOMMIT connection.
+    Scheduled weekly; also operator-triggerable from Settings → Maintenance.
+    """
+    engine = get_sync_engine()
+    done = []
+    with engine.connect().execution_options(isolation_level="AUTOCOMMIT") as conn:
+        for table in VACUUM_TABLES:
+            conn.exec_driver_sql(f"VACUUM (ANALYZE) {table}")
+            done.append(table)
+    log.info("vacuum_analyze complete: %s", done)
+    return {"vacuumed": done}