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

nginx/start.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+set -e
+
+if [ "${NGINX_CACHE_ENABLED:-false}" = "true" ]; then
+    NGINX_CACHE_CONFIG='        proxy_cache app_cache;
+        proxy_cache_bypass $http_cache_control;
+        proxy_cache_valid 200 1m;
+        proxy_cache_use_stale error timeout updating;
+        proxy_cache_lock on;
+        add_header X-Cache-Status $upstream_cache_status;'
+fi
+
+export NGINX_CACHE_CONFIG
+export NGINX_CACHE_MAX_SIZE="${NGINX_CACHE_MAX_SIZE:-1g}"
+
+envsubst '${NGINX_CACHE_CONFIG} ${NGINX_CACHE_MAX_SIZE}' \
+    < /etc/nginx/nginx.conf.template \
+    > /etc/nginx/nginx.conf
+
+exec nginx -g 'daemon off;'