✨(backend) add file upload feature & tests

For the coming features we will need to store files on the meet side.
(for instance user backgrounds).

This commits adds a new Model to manage files, and the associated
serializers & viewsets. All are tested.

This work was heavily inspired by the work done by our friends at
https://github.com/suitenumerique/drive
It build on the same architecture design (upload directly to S3 but
download goes through our proxy), but model is much much simplier
(no folders, no file sharing, etc.).

src/backend/core/api/permissions.py

@@ -1,5 +1,7 @@
 """Permission handlers for the Meet core app."""
 
+from django.http import Http404
+
 from rest_framework import permissions
 
 from ..models import RoleChoices
@@ -106,3 +108,23 @@ class HasLiveKitRoomAccess(permissions.BasePermission):
         if not request.auth or not hasattr(request.auth, "video"):
             return False
         return request.auth.video.room == str(obj.id)
+
+
+class FilePermission(IsAuthenticated):
+    """
+    Permissions applying to the file API endpoint.
+    Handling soft deletions specificities
+    """
+
+    def has_object_permission(self, request, view, obj):
+        """
+        Return a 404 on deleted files or if the user is not the owner
+        """
+
+        if obj.deleted_at is not None or obj.hard_deleted_at is not None:
+            raise Http404
+
+        if obj.creator != request.user:
+            raise Http404
+
+        return obj.get_abilities(request.user).get(view.action, False)