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>

backend/app/limiter.py

@@ -0,0 +1,4 @@
+from slowapi import Limiter
+from slowapi.util import get_remote_address
+
+limiter = Limiter(key_func=get_remote_address)