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/config.py

@@ -8,6 +8,7 @@ class Settings(BaseSettings):
     algorithm: str = "HS256"
     admin_username: str
     admin_password: str
+    allowed_origins: str = "http://localhost"
 
     class Config:
         env_file = ".env"