feat(api): implement OpenAPI/REST API for file conversions

- Add comprehensive REST API at /api/v1 with modular structure
- Implement authentication endpoints (register, login, logout, me)
- Add converter listing and format discovery endpoints
- Create job management and file download endpoints
- Add health check endpoint for monitoring
- Set up CORS support for browser-based API clients
- Create API middleware for JWT authentication
- Add environment variables for API configuration
- Include comprehensive API documentation and test script

Infrastructure:
- Enhanced CI/CD workflow for custom Docker registries
- Docker Compose setup with dev, prod, and monitoring profiles
- Claude.ai integration files for development workflow
- Environment-based configuration with .env.development

Known limitations:
- JWT authentication context needs fixing (using ALLOW_UNAUTHENTICATED=true)
- Swagger UI temporarily disabled due to composition error
- File upload endpoint needs multipart/form-data support

The API code is isolated in src/api/ directory to maintain separation
from the existing codebase, making it easy to maintain or contribute back.

.claude/build.md

@@ -0,0 +1,50 @@
+# Build and Test Docker Image
+
+Please build and test the Docker image with these steps:
+
+1. **Build the Docker image**:
+   ```bash
+   docker build -t convertx-openapi:local .
+   ```
+
+2. **Verify the build**:
+   - Check image size: `docker images convertx-openapi:local`
+   - Inspect layers: `docker history convertx-openapi:local`
+
+3. **Run container locally**:
+   ```bash
+   docker run -d \
+     --name convertx-test \
+     -p 3000:3000 \
+     -v $(pwd)/data:/app/data \
+     -e JWT_SECRET=test-secret \
+     -e NODE_ENV=production \
+     convertx-openapi:local
+   ```
+
+4. **Test the application**:
+   - Check logs: `docker logs convertx-test`
+   - Test health: `curl http://localhost:3000`
+   - Verify converters load properly
+
+5. **Multi-platform build** (if needed):
+   ```bash
+   docker buildx build \
+     --platform linux/amd64,linux/arm64 \
+     -t convertx-openapi:local \
+     .
+   ```
+
+6. **Push to custom registry** (if configured):
+   ```bash
+   docker tag convertx-openapi:local ${DOCKER_REGISTRY}/convertx:latest
+   docker push ${DOCKER_REGISTRY}/convertx:latest
+   ```
+
+7. **Cleanup**:
+   ```bash
+   docker stop convertx-test
+   docker rm convertx-test
+   ```
+
+Please execute these steps and report any issues.