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

.dockerignore

@@ -0,0 +1,18 @@
+.git
+.gitignore
+__pycache__
+*.pyc
+.pytest_cache
+screenshots
+devplace.db
+devplace.db-shm
+devplace.db-wal
+.env
+.venv
+node_modules
+.opencode
+locustfile.py
+locustresults.html
+*.md
+tests
+.DS_Store