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>

frontend/src/stores/dbMaintenance.js

@@ -0,0 +1,25 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import { useApi } from '../composables/useApi.js'
+
+export const useDbMaintenanceStore = defineStore('dbMaintenance', () => {
+  const api = useApi()
+  const tables = ref([])
+  const loading = ref(false)
+
+  async function loadStats() {
+    loading.value = true
+    try {
+      const body = await api.get('/api/admin/maintenance/db-stats')
+      tables.value = body.tables || []
+    } finally {
+      loading.value = false
+    }
+  }
+
+  async function runVacuum() {
+    return await api.post('/api/admin/maintenance/vacuum')
+  }
+
+  return { tables, loading, loadStats, runVacuum }
+})