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/db/models.py

@@ -6,6 +6,9 @@ from datetime import datetime
 
 from app.db.session import Base
 
+# ---------------------------------------------------------
+# Modelo de investigador
+# ---------------------------------------------------------
 
 class Researcher(Base):
     __tablename__ = "researchers"
@@ -18,6 +21,9 @@ class Researcher(Base):
 
     publications = relationship("Publication", back_populates="researcher", cascade="all, delete-orphan")
 
+# ---------------------------------------------------------
+# Modelo de publicación
+# ---------------------------------------------------------
 
 class Publication(Base):
     __tablename__ = "publications"
@@ -65,6 +71,9 @@ class Publication(Base):
     # Legacy: descargado global (deprecado). Mantener por compatibilidad de DB.
     downloaded = Column(Boolean, nullable=False, default=False)
 
+# ---------------------------------------------------------
+# Modelo de descarga de publicación
+# ---------------------------------------------------------
 
 class PublicationDownload(Base):
     """