feat(db): scheduled DB maintenance — daily targeted VACUUM (ANALYZE)

Adds a daily off-hours VACUUM (ANALYZE) over the high-churn tables the
retention/purge sweeps churn (app_logs, notifications, token tables, notes,
note_versions), on top of Postgres autovacuum, to reclaim bloat left by the
nightly bulk DELETEs and keep planner stats fresh.

- services/db_maintenance.py: run_maintenance() over a closed table allowlist
  via an AUTOCOMMIT connection (VACUUM can't run in a txn); per-table summary
  persisted as the db_maintenance_last_run admin setting.
- services/db_maintenance_scheduler.py: BackgroundScheduler cron (default
  04:00 UTC, after the 03:30 trash purge); enabled-gate checked at fire time;
  live reschedule on hour change. Wired into app.py start/stop.
- routes/admin.py: admin-only GET/PUT /api/admin/db-maintenance + POST /run.
- settings.py: set_admin_setting() (write-side of get_admin_setting) for
  out-of-request writes.
- SettingsView.vue: admin 'Database maintenance' card — enable toggle, run-hour
  (UTC), Run-now, last-run summary.
- Tests: allowlist is closed, VACUUM issued per table, one failure doesn't
  abort the rest, summary persisted; route/scheduler/service surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scribe/app.py

@@ -187,6 +187,15 @@ def create_app() -> Quart:
         from scribe.services.trash_scheduler import start_trash_scheduler
         start_trash_scheduler(asyncio.get_running_loop())
 
+        # DB maintenance scheduler (daily targeted VACUUM ANALYZE, default 04:00 UTC)
+        from scribe.services.db_maintenance_scheduler import (
+            get_maintenance_hour,
+            start_db_maintenance_scheduler,
+        )
+        start_db_maintenance_scheduler(
+            asyncio.get_running_loop(), await get_maintenance_hour()
+        )
+
         # Diagnostic instrumentation — heartbeat, signal handlers, asyncio
         # exception hook. Cheap (~1 log line/min), high diagnostic value when
         # the app crashes mysteriously. See services/diagnostics.py.
@@ -203,6 +212,10 @@ def create_app() -> Quart:
         stop_version_pinning_scheduler()
         from scribe.services.trash_scheduler import stop_trash_scheduler
         stop_trash_scheduler()
+        from scribe.services.db_maintenance_scheduler import (
+            stop_db_maintenance_scheduler,
+        )
+        stop_db_maintenance_scheduler()
         from scribe.services.diagnostics import stop_diagnostics
         stop_diagnostics()