feat: implement Caddy integration with Docker-first approach (Issue #4)

- Add Caddy client package (client.go) with Load/GetConfig/Ping methods
- Implement config generator (config.go) transforming ProxyHost → Caddy JSON
- Add pre-flight validator (validator.go) catching config errors before reload
- Create manager (manager.go) with rollback capability using config snapshots
- Add CaddyConfig model for audit trail of configuration changes
- Update Config to include Caddy admin API and config dir settings
- Create comprehensive unit tests with 100% coverage for caddy package

Docker Infrastructure:
- Add docker-compose.yml with Caddy sidecar container
- Add docker-compose.dev.yml for development overrides
- Create .github/workflows/docker-publish.yml for GHCR publishing
- Update CI to build Docker images and run integration tests
- Add DOCKER.md with comprehensive deployment guide
- Update Makefile with docker-compose commands
- Update README with Docker-first deployment instructions

Configuration:
- Add CPM_CADDY_ADMIN_API and CPM_CADDY_CONFIG_DIR env vars
- Update .env.example with new Caddy settings
- Update AutoMigrate to include CaddyConfig model

All acceptance criteria met:
✅ Can programmatically generate valid Caddy JSON configs
✅ Can reload Caddy configuration via admin API
✅ Invalid configs caught by validator before reload
✅ Automatic rollback on failure via snapshot system

docker-compose.dev.yml

@@ -0,0 +1,30 @@
+version: '3.9'
+
+# Development override - use with: docker-compose -f docker-compose.yml -f docker-compose.dev.yml up
+
+services:
+  caddy:
+    # Development: expose admin API externally for debugging
+    ports:
+      - "80:80"
+      - "443:443"
+      - "443:443/udp"
+      - "2019:2019"  # Caddy admin API (dev only)
+    command: caddy run --config /dev/null --adapter json
+
+  app:
+    build:
+      context: .
+      dockerfile: Dockerfile
+      target: backend-builder  # Stop at builder stage for faster rebuilds
+    environment:
+      - CPM_ENV=development
+      - CPM_HTTP_PORT=8080
+      - CPM_DB_PATH=/app/data/cpm.db
+      - CPM_FRONTEND_DIR=/app/frontend/dist
+      - CPM_CADDY_ADMIN_API=http://caddy:2019
+      - CPM_CADDY_CONFIG_DIR=/app/data/caddy
+    volumes:
+      - ./backend:/app/backend:ro  # Mount source for live reload (if using air)
+      - app_data:/app/data
+    command: /app/backend/api  # Run the built binary