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/database.py

@@ -4,7 +4,7 @@ from sqlalchemy.orm import DeclarativeBase
 from app.config import settings
 
 
-engine = create_async_engine(settings.database_url, echo=False, pool_pre_ping=True, pool_recycle=1800)
+engine = create_async_engine(settings.database_url, echo=False, pool_recycle=1800)
 AsyncSessionLocal = async_sessionmaker(engine, expire_on_commit=False)