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/api/admin.py

@@ -19,7 +19,7 @@ from __future__ import annotations
 import hashlib
 
 from quart import Blueprint, jsonify, request
-from sqlalchemy import select
+from sqlalchemy import select, text
 
 from ..extensions import get_session
 from ..models import Artist
@@ -222,3 +222,49 @@ async def tags_purge_legacy():
             lambda sync_sess: purge_legacy_tags(sync_sess, dry_run=dry_run)
         )
     return jsonify(result)
+
+
+@admin_bp.route("/maintenance/db-stats", methods=["GET"])
+async def db_stats():
+    """Per-table bloat readout (pg_stat_user_tables) for the high-churn tables
+    so the operator can see when a VACUUM is worth running."""
+    from ..tasks.maintenance import VACUUM_TABLES
+
+    wanted = set(VACUUM_TABLES)
+    async with get_session() as session:
+        rows = (await session.execute(text(
+            "SELECT relname, n_live_tup, n_dead_tup, last_vacuum, "
+            "last_autovacuum, last_analyze FROM pg_stat_user_tables"
+        ))).all()
+
+    def _iso(v):
+        return v.isoformat() if v is not None else None
+
+    out = []
+    for r in rows:
+        if r.relname not in wanted:
+            continue
+        live = r.n_live_tup or 0
+        dead = r.n_dead_tup or 0
+        total = live + dead
+        out.append({
+            "table": r.relname,
+            "live": live,
+            "dead": dead,
+            "dead_pct": round(100 * dead / total, 1) if total else 0.0,
+            "last_vacuum": _iso(r.last_vacuum),
+            "last_autovacuum": _iso(r.last_autovacuum),
+            "last_analyze": _iso(r.last_analyze),
+        })
+    out.sort(key=lambda t: t["dead"], reverse=True)
+    return jsonify({"tables": out})
+
+
+@admin_bp.route("/maintenance/vacuum", methods=["POST"])
+async def trigger_vacuum():
+    """Operator-triggered VACUUM (ANALYZE) over the high-churn tables — the
+    same maintenance-queue task the weekly Beat schedule runs."""
+    from ..tasks.maintenance import vacuum_analyze
+
+    vacuum_analyze.delay()
+    return jsonify({"status": "queued"}), 202