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/requirements.txt

@@ -9,3 +9,4 @@ bcrypt==4.0.1
 python-multipart==0.0.20
 pytz==2024.2
 email-validator==2.2.0
+slowapi==0.1.9