🦺(backend) strengthen API validation for recording options

Improve validation of parameters accepted when starting a
recording to prevent unsupported or unexpected values.

Language validation will be further tightened to only accept
languages supported by the transcribe microservice.

Add extensive API validation tests to cover these scenarios.

src/backend/core/api/viewsets.py

@@ -296,12 +296,14 @@ class RoomViewSet(
             )
 
         mode = serializer.validated_data["mode"]
-        options = serializer.validated_data["options"]
+        options = serializer.validated_data.get("options")
         room = self.get_object()
 
         # May raise exception if an active or initiated recording already exist for the room
         recording = models.Recording.objects.create(
-            room=room, mode=mode, options=options
+            room=room,
+            mode=mode,
+            options=options.model_dump(exclude_none=True) if options else {},
         )
 
         models.RecordingAccess.objects.create(