✨(backend) add Traefik reverse proxy support for media-auth

Adds support for serving media behind Traefik, which currently cannot work
at all.

The media-auth subrequest views read the original request URL from a
hardcoded HTTP_X_ORIGINAL_URL header. That header is an nginx-ingress
convention. Traefik's ForwardAuth middleware sends X-Forwarded-Uri instead
and has no mechanism to emit X-Original-URL, so behind Traefik every
recording download and file attachment is rejected with a bare 403 --
indistinguishable from a legitimate permission denial, which makes it
painful to diagnose.

Add MEDIA_AUTH_ORIGINAL_URL_HEADER, defaulting to HTTP_X_ORIGINAL_URL so
existing nginx-ingress deployments are unaffected. Traefik deployments set
it to HTTP_X_FORWARDED_URI. It is used in both places that resolve the
header: RecordingViewSet._auth_get_original_url and the file attachment
_authorize_subrequest. The log message on a missing header now names the
header actually expected, which is what makes the failure diagnosable.

This mirrors the setting the sibling Docs project already exposes
(suitenumerique/docs, MEDIA_AUTH_ORIGINAL_URL_HEADER) for the same reason.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/backend/core/api/viewsets.py

@@ -1076,9 +1076,10 @@ class RecordingViewSet(
 
     def _auth_get_original_url(self, request):
         """
-        Extracts and parses the original URL from the "HTTP_X_ORIGINAL_URL" header.
+        Extracts and parses the original URL from the configured header.
         Raises PermissionDenied if the header is missing.
-        The original url is passed by nginx in the "HTTP_X_ORIGINAL_URL" header.
+        The original url is passed by the reverse proxy in the header named by the
+        MEDIA_AUTH_ORIGINAL_URL_HEADER setting, which defaults to "HTTP_X_ORIGINAL_URL".
         See corresponding ingress configuration in Helm chart and read about the
         nginx.ingress.kubernetes.io/auth-url annotation to understand how the Nginx ingress
         is configured to do this.
@@ -1088,9 +1089,13 @@ class RecordingViewSet(
         reasons.
         """
         # Extract the original URL from the request header
-        original_url = request.META.get("HTTP_X_ORIGINAL_URL")
+        original_url = request.META.get(settings.MEDIA_AUTH_ORIGINAL_URL_HEADER)
         if not original_url:
-            logger.warning("Missing HTTP_X_ORIGINAL_URL header in subrequest")
+            logger.warning(
+                "Missing %s header in subrequest. Set MEDIA_AUTH_ORIGINAL_URL_HEADER "
+                "to the header your reverse proxy sends.",
+                settings.MEDIA_AUTH_ORIGINAL_URL_HEADER,
+            )
             raise drf_exceptions.PermissionDenied()
 
         logger.debug("Original url: '%s'", original_url)
@@ -1415,7 +1420,8 @@ class FileViewSet(
         Authorize access based on the original URL of an Nginx subrequest
         and user permissions. Returns a dictionary of URL parameters if authorized.
 
-        The original url is passed by nginx in the "HTTP_X_ORIGINAL_URL" header.
+        The original url is passed by the reverse proxy in the header named by the
+        MEDIA_AUTH_ORIGINAL_URL_HEADER setting, which defaults to "HTTP_X_ORIGINAL_URL".
         See corresponding ingress configuration in Helm chart and read about the
         nginx.ingress.kubernetes.io/auth-url annotation to understand how the Nginx ingress
         is configured to do this.
@@ -1434,9 +1440,13 @@ class FileViewSet(
         - PermissionDenied if authorization fails.
         """
         # Extract the original URL from the request header
-        original_url = request.META.get("HTTP_X_ORIGINAL_URL")
+        original_url = request.META.get(settings.MEDIA_AUTH_ORIGINAL_URL_HEADER)
         if not original_url:
-            logger.warning("Missing HTTP_X_ORIGINAL_URL header in subrequest")
+            logger.warning(
+                "Missing %s header in subrequest. Set MEDIA_AUTH_ORIGINAL_URL_HEADER "
+                "to the header your reverse proxy sends.",
+                settings.MEDIA_AUTH_ORIGINAL_URL_HEADER,
+            )
             raise drf_exceptions.PermissionDenied()
 
         parsed_url = urlparse(original_url)