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/schema/export.py

@@ -0,0 +1,23 @@
+"""
+Schemas de los endpoints de export.
+
+El backend recibe `pub_ids` como UUIDs en formato string. Pydantic ya los
+valida y convierte; aquí además aplicamos un tope de tamaño para impedir
+peticiones gigantes.
+"""
+
+from __future__ import annotations
+
+from typing import List
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+from app.core.config import settings
+
+
+class PublicationIdsRequestSchema(BaseModel):
+    pub_ids: List[UUID] = Field(
+        min_length=1,
+        max_length=settings.MAX_PUB_IDS_BATCH,
+    )