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/services/settings.py

@@ -26,6 +26,23 @@ async def get_admin_setting(key: str, default: str = "") -> str:
         return setting.value if setting and setting.value else default
 
 
+async def set_admin_setting(key: str, value: str) -> None:
+    """Write an instance-global setting onto the first admin account.
+
+    The write-side counterpart to get_admin_setting, for non-per-user settings
+    written outside a request context (e.g. a scheduler persisting its last-run
+    summary) where get_current_user_id() isn't available.
+    """
+    async with async_session() as session:
+        admin_id = (
+            await session.execute(
+                select(User.id).where(User.role == "admin").order_by(User.id)
+            )
+        ).scalars().first()
+    if admin_id is not None:
+        await set_setting(admin_id, key, value)
+
+
 async def get_setting(user_id: int, key: str, default: str = "") -> str:
     async with async_session() as session:
         result = await session.execute(