✨(backend) implement recording expiration mechanism

Add expiration system for recordings.

Include option for users to set recordings as permanent (no expiration)
which is the default behavior.

System only calculates expiration dates and tracks status - actual deletion
is handled by Minio bucket lifecycle policies, not by application code.

src/backend/core/api/__init__.py

@@ -40,6 +40,7 @@ def get_frontend_configuration(request):
         "recording": {
             "is_enabled": settings.RECORDING_ENABLE,
             "available_modes": settings.RECORDING_WORKER_CLASSES.keys(),
+            "expiration_days": settings.RECORDING_EXPIRATION_DAYS,
         },
     }
     frontend_configuration.update(settings.FRONTEND_CONFIGURATION)

src/backend/core/api/serializers.py

@@ -159,7 +159,17 @@ class RecordingSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.Recording
-        fields = ["id", "room", "created_at", "updated_at", "status", "mode", "key"]
+        fields = [
+            "id",
+            "room",
+            "created_at",
+            "updated_at",
+            "status",
+            "mode",
+            "key",
+            "is_expired",
+            "expired_at",
+        ]
         read_only_fields = fields