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

starpunk/__init__.py

@@ -52,6 +52,54 @@ def create_app(config=None):
             return {"error": "Internal server error"}, 500
         return render_template("500.html"), 500
 
+    # Health check endpoint for containers and monitoring
+    @app.route("/health")
+    def health_check():
+        """
+        Health check endpoint for containers and monitoring
+
+        Returns:
+            JSON with status and basic info
+
+        Response codes:
+            200: Application healthy
+            500: Application unhealthy
+
+        Checks:
+            - Database connectivity
+            - File system access
+            - Basic application state
+        """
+        from flask import jsonify
+        import os
+
+        try:
+            # Check database connectivity
+            from starpunk.database import get_db
+
+            db = get_db(app)
+            db.execute("SELECT 1").fetchone()
+            db.close()
+
+            # Check filesystem access
+            data_path = app.config.get("DATA_PATH", "data")
+            if not os.path.exists(data_path):
+                raise Exception("Data path not accessible")
+
+            return (
+                jsonify(
+                    {
+                        "status": "healthy",
+                        "version": app.config.get("VERSION", __version__),
+                        "environment": app.config.get("ENV", "unknown"),
+                    }
+                ),
+                200,
+            )
+
+        except Exception as e:
+            return jsonify({"status": "unhealthy", "error": str(e)}), 500
+
     return app