feat: enhance backend security and configuration

- Updated Dockerfile to improve security with a non-root user and added health checks.
- Modified docker-compose.yml to set containers as read-only, restrict ports to localhost, and implement health checks.
- Enhanced .env.example with additional environment variables for security and configuration.
- Improved FastAPI application with middleware for security headers, CORS, and body size limits.
- Refactored authentication flow in auth.py to include state validation and improved error handling.
- Added rate limiting to various endpoints to prevent abuse.
- Updated researcher and publication handling to ensure better validation and error management.

backend/app/services/normalizer.py

@@ -1,5 +1,8 @@
 from typing import List
 
+# ---------------------------------------------------------
+# Función auxiliar: obtener valor de un diccionario
+# ---------------------------------------------------------
 
 def _get(d: dict | None, *keys, default=None):
     cur = d or {}
@@ -11,6 +14,9 @@ def _get(d: dict | None, *keys, default=None):
             return default
     return cur
 
+# ---------------------------------------------------------
+# Clase de normalización de publicaciones
+# ---------------------------------------------------------
 
 class PublicationNormalizer:
     @staticmethod