Align Sinbad2 HTTPS deployment with orcid2sword reverse-proxy pattern.

This adds nginx dual-path routing, forwarded proxy headers, Uvicorn proxy-headers, production security settings, and deployment docs for https://sinbad2.ujaen.es/generadorexamenesllm/.

backend/app/core/security_headers.py

@@ -0,0 +1,30 @@
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
+
+from app.core.config import Settings
+
+
+class SecurityHeadersMiddleware(BaseHTTPMiddleware):
+    """Cabeceras de seguridad; HSTS en producción o cuando el proxy indica HTTPS."""
+
+    def __init__(self, app: object, settings: Settings) -> None:
+        super().__init__(app)
+        self._settings = settings
+
+    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
+        response = await call_next(request)
+
+        response.headers.setdefault("X-Content-Type-Options", "nosniff")
+        response.headers.setdefault("X-Frame-Options", "SAMEORIGIN")
+        response.headers.setdefault("Referrer-Policy", "strict-origin-when-cross-origin")
+
+        forwarded_proto = request.headers.get("x-forwarded-proto", request.url.scheme)
+        is_https = forwarded_proto == "https" or request.url.scheme == "https"
+
+        if is_https or self._settings.is_production:
+            hsts = f"max-age={self._settings.security_hsts_seconds}"
+            if self._settings.is_production:
+                hsts += "; includeSubDomains"
+            response.headers.setdefault("Strict-Transport-Security", hsts)
+
+        return response