Fix aiomysql pool_pre_ping crash, add rate limiting and security hardening

Remove pool_pre_ping=True to fix startup crash caused by aiomysql 0.2.0
async adapter requiring a reconnect argument SQLAlchemy does not pass.
Add slowapi rate limiting, structured logging, CORS config, backend
health check, and nginx security headers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -18,10 +18,17 @@ services:
       - ACCESS_TOKEN_EXPIRE_MINUTES=${ACCESS_TOKEN_EXPIRE_MINUTES:-480}
       - ADMIN_USERNAME=${ADMIN_USERNAME}
       - ADMIN_PASSWORD=${ADMIN_PASSWORD}
+      - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-http://localhost}
     depends_on:
       db:
         condition: service_healthy
     restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
 
   db:
     image: mysql:8