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>

docker-compose.yml

@@ -19,6 +19,10 @@ services:
       - DEV_MODE=${DEV_MODE:-false}
       # Optional: Enable debug mode
       - DEBUG=${DEBUG:-false}
+      # Database path (inside container)
+      - DATABASE_PATH=/app/data/kuma_strapper.db
+    volumes:
+      - ./data:/app/data
     restart: unless-stopped
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:5000/api/health"]