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

.github/workflows/docker-publish.yml

@@ -0,0 +1,74 @@
+name: Docker Build & Publish
+
+on:
+  push:
+    branches:
+      - main
+      - development
+    tags:
+      - 'v*.*.*'
+  pull_request:
+    branches:
+      - main
+      - development
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            # Tag 'latest' for main branch
+            type=raw,value=latest,enable={{is_default_branch}}
+            # Tag 'development' for development branch
+            type=raw,value=development,enable=${{ github.ref == 'refs/heads/development' }}
+            # Semver tags for version releases (v1.0.0 -> 1.0.0, 1.0, 1)
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            # SHA for all builds
+            type=sha,prefix={{branch}}-
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            BUILD_DATE=${{ github.event.head_commit.timestamp }}
+            VCS_REF=${{ github.sha }}
+            VERSION=${{ steps.meta.outputs.version }}
+
+      - name: Image digest
+        run: echo ${{ steps.build-and-push.outputs.digest }}