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

.containerignore

@@ -0,0 +1,78 @@
+# Container Build Exclusions
+# Exclude files not needed in production container image
+
+# Git
+.git
+.gitignore
+.gitattributes
+
+# Python
+__pycache__
+*.pyc
+*.pyo
+*.pyd
+.Python
+*.so
+*.egg
+*.egg-info
+dist
+build
+.pytest_cache
+.coverage
+htmlcov
+.tox
+.hypothesis
+
+# Virtual environments
+venv
+env
+.venv
+.env.local
+
+# Development data
+data
+container-data
+*.db
+*.db-journal
+
+# IDE
+.vscode
+.idea
+*.swp
+*.swo
+*~
+.DS_Store
+
+# Documentation (optional - include if needed for offline docs)
+docs
+*.md
+!README.md
+
+# Tests (not needed in production)
+tests
+.pytest_cache
+
+# Development scripts
+dev_auth.py
+test_*.py
+
+# Container files
+Containerfile
+compose.yaml
+.containerignore
+docker-compose.yml
+Dockerfile
+
+# CI/CD
+.github
+.gitlab-ci.yml
+.travis.yml
+
+# Logs
+*.log
+logs
+
+# Temporary files
+tmp
+temp
+*.tmp