Add local SQLite database and sync with Uptime Kuma

Features:
- SQLite database to track monitors and hosts locally
- Uses Uptime Kuma tags to mark monitors as managed by Kuma Strapper
- Sync on startup, before each scan, and on-demand via API
- Shows existing monitors when re-scanning a host

New files:
- backend/services/database.py - SQLite database service
- backend/services/sync.py - Sync service for Uptime Kuma reconciliation

API endpoints:
- POST /api/sync - Full sync with Uptime Kuma
- POST /api/sync/host/<hostname> - Sync specific host
- GET /api/hosts - List tracked hosts
- GET /api/hosts/<hostname>/monitors - Get monitors for host
- GET /api/monitors/tracked - Get all tracked monitors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.js

@@ -81,4 +81,13 @@ export const api = {
     body: JSON.stringify({ username, password, totp }),
   }),
   kumaLogout: () => fetchApi('/kuma/logout', { method: 'POST' }),
+
+  // Sync
+  triggerSync: () => fetchApi('/sync', { method: 'POST' }),
+  syncHost: (hostname) => fetchApi(`/sync/host/${encodeURIComponent(hostname)}`, { method: 'POST' }),
+
+  // Hosts and tracked monitors
+  getHosts: () => fetchApi('/hosts'),
+  getHostMonitors: (hostname) => fetchApi(`/hosts/${encodeURIComponent(hostname)}/monitors`),
+  getTrackedMonitors: () => fetchApi('/monitors/tracked'),
 };