feat: implement full CI/CD pipeline with GitHub Actions and Flux

- Add GitHub Actions workflow to build and push Docker images to GHCR
- Add Dockerfile for multi-stage builds
- Update deployment to use GHCR images
- Add GHCR authentication secrets
- Configure Flux ImageRepository, ImagePolicy, and ImageUpdateAutomation
- Remove init container approach in favor of proper Docker builds

Auto-deploy flow:
1. Push to master triggers GitHub Actions
2. GitHub Actions builds image with commit hash tag
3. Image pushed to ghcr.io/unchainedio/manoon-headless
4. Flux ImageRepository detects new image
5. Flux ImageUpdateAutomation updates kustomization.yaml
6. Flux Kustomization applies new deployment
7. Kubernetes restarts pods with new image

Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /app
 
 # Copy package files
 COPY package*.json ./
-RUN npm ci --prefer-offline --no-audit
+RUN npm install --prefer-offline --no-audit
 
 # Copy source and build
 COPY . .