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/test/dbMaintenance.spec.js

@@ -0,0 +1,44 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useDbMaintenanceStore } from '../src/stores/dbMaintenance.js'
+
+function stubFetch(handler) {
+  globalThis.fetch = vi.fn(async (url, init) => {
+    const { status, body } = handler(url, init)
+    return {
+      ok: status >= 200 && status < 300,
+      status,
+      statusText: String(status),
+      text: async () => (body == null ? '' : JSON.stringify(body)),
+    }
+  })
+}
+
+describe('dbMaintenance store', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+  afterEach(() => vi.restoreAllMocks())
+
+  it('loadStats populates the bloat table', async () => {
+    const s = useDbMaintenanceStore()
+    stubFetch(() => ({
+      status: 200,
+      body: { tables: [{ table: 'image_record', live: 5, dead: 1, dead_pct: 16.7, last_vacuum: null }] },
+    }))
+    await s.loadStats()
+    expect(s.tables).toHaveLength(1)
+    expect(s.tables[0].table).toBe('image_record')
+  })
+
+  it('runVacuum POSTs the trigger endpoint', async () => {
+    const s = useDbMaintenanceStore()
+    const calls = []
+    stubFetch((url, init) => {
+      calls.push({ url, init })
+      return { status: 202, body: { status: 'queued' } }
+    })
+    await s.runVacuum()
+    const c = calls.at(-1)
+    expect(c.url).toContain('/api/admin/maintenance/vacuum')
+    expect(c.init.method).toBe('POST')
+  })
+})