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

.env.example

@@ -64,6 +64,36 @@ FLASK_DEBUG=1
 # Flask secret key (falls back to SESSION_SECRET if not set)
 FLASK_SECRET_KEY=
 
+# =============================================================================
+# RSS FEED CONFIGURATION
+# =============================================================================
+
+# Maximum number of items in RSS feed (default: 50)
+FEED_MAX_ITEMS=50
+
+# Feed cache duration in seconds (default: 300 = 5 minutes)
+FEED_CACHE_SECONDS=300
+
+# =============================================================================
+# CONTAINER CONFIGURATION
+# =============================================================================
+
+# Application version (for health check endpoint)
+VERSION=0.6.0
+
+# Environment: development or production
+ENVIRONMENT=production
+
+# Number of Gunicorn workers (default: 4)
+# Recommendation: (2 x CPU cores) + 1
+WORKERS=4
+
+# Worker timeout in seconds (default: 30)
+WORKER_TIMEOUT=30
+
+# Max requests per worker before restart (prevents memory leaks)
+MAX_REQUESTS=1000
+
 # =============================================================================
 # DEVELOPMENT OPTIONS
 # =============================================================================