chore: add docker infrastructure, gists feature, attachment system, and admin CLI tools

- Add .dockerignore, Dockerfile, and docker compose targets to Makefile for containerized deployment
- Implement gists router with CRUD operations, database schema, and polymorphic comment/vote reuse
- Create attachment upload system with thumbnail generation, MIME detection, and storage path management
- Add attachments_prune CLI command to clean orphaned attachment records and files
- Introduce rate limiting middleware with 60 requests per minute window
- Add custom 404 and 500 error handlers with SEO-optimized template responses
- Extend database initialization with gists and attachments indexes plus upload site settings defaults
- Update load_comments to include attachment mapping for comment resources
- Register gists and uploads routers in main application and update AGENTS.md documentation

Makefile

@@ -53,3 +53,20 @@ clean:
 	find . -type f -name '*.pyc' -delete
 	rm -rf devplacepy.egg-info
 	rm -rf .venv
+
+.PHONY: docker-build docker-up docker-down docker-logs docker-clean
+
+docker-build:
+	docker compose build
+
+docker-up:
+	docker compose up -d
+
+docker-down:
+	docker compose down
+
+docker-logs:
+	docker compose logs -f
+
+docker-clean:
+	docker compose down -v