✨(backend) add two new endpoints to start and stop a recording

The LiveKit egress worker interactions are proxied through the backend for
security reasons. Allowing clients to directly use tokens with sufficient
grants to start recordings could lead to misuse, enabling users to spam the
egress worker API and potentially initiate a DDOS attack on the egress
service. To prevent this, only users with room-specific privileges can
initiate recordings.

We make sure only one recording at the time can be made on a room.

The requested recording mode is stored so it can be referenced later when
the recording is saved, triggering a callback action as needed.

A feature flag was also introduced for this capability; while this is a simple
approach, a more robust system for managing feature flags could be valuable
long-term. For now, KISS (Keep It Simple, Stupid) applies.

The viewset endpoints were designed to be as straightforward as possible—
let me know if anything can be improved.

src/backend/core/api/serializers.py

@@ -156,3 +156,25 @@ class RecordingSerializer(serializers.ModelSerializer):
         model = models.Recording
         fields = ["id", "room", "created_at", "updated_at", "status"]
         read_only_fields = fields
+
+
+class StartRecordingSerializer(serializers.Serializer):
+    """Validate start recording requests."""
+
+    mode = serializers.ChoiceField(
+        choices=models.RecordingModeChoices.choices,
+        required=True,
+        error_messages={
+            "required": "Recording mode is required.",
+            "invalid_choice": "Invalid recording mode. Choose between "
+            "screen_recording or transcript.",
+        },
+    )
+
+    def create(self, validated_data):
+        """Not implemented as this is a validation-only serializer."""
+        raise NotImplementedError("StartRecordingSerializer is validation-only")
+
+    def update(self, instance, validated_data):
+        """Not implemented as this is a validation-only serializer."""
+        raise NotImplementedError("StartRecordingSerializer is validation-only")