feat: add production container support with health check endpoint

Implements Phase 5 containerization specification:
- Add /health endpoint for container monitoring
- Create multi-stage Containerfile (Podman/Docker compatible)
- Add compose.yaml for orchestration
- Add Caddyfile.example for reverse proxy (auto-HTTPS)
- Add nginx.conf.example as alternative
- Update .env.example with container and RSS feed variables
- Add gunicorn WSGI server to requirements.txt

Container features:
- Multi-stage build for smaller image size
- Non-root user (starpunk:1000)
- Health check with database connectivity test
- Volume mount for data persistence
- Resource limits and logging configuration
- Security headers and HTTPS configuration examples

Health check endpoint:
- Tests database connectivity
- Verifies filesystem access
- Returns JSON with status, version, and environment

Following Phase 5 design in docs/designs/phase-5-rss-and-container.md

requirements.txt

@@ -4,6 +4,9 @@
 # Web Framework
 Flask==3.0.*
 
+# WSGI Server (Production)
+gunicorn==21.2.*
+
 # Content Processing
 markdown==3.5.*