♻️(backend) replace Django permissions with feature flag decorator

Refactor feature flag mechanism from Django permission classes to custom
decorator that returns 404 Not Found when features are disabled instead
of exposing API structure through permission errors.

Improves security by preventing information disclosure about disabled
features and provides more appropriate response semantics. Custom
decorator approach is better suited for feature toggling than Django's
permission system which is designed for authorization.

src/backend/core/api/feature_flag.py

@@ -0,0 +1,45 @@
+"""Feature flag handler for the Meet core app."""
+
+from functools import wraps
+
+from django.conf import settings
+from django.http import Http404
+
+
+class FeatureFlag:
+    """Check if features are enabled and return error responses."""
+
+    FLAGS = {
+        "recording": "RECORDING_ENABLE",
+        "storage_event": "RECORDING_STORAGE_EVENT_ENABLE",
+        "subtitle": "ROOM_SUBTITLE_ENABLED",
+    }
+
+    @classmethod
+    def flag_is_active(cls, flag_name):
+        """Check if a feature flag is active."""
+
+        setting_name = cls.FLAGS.get(flag_name)
+
+        if setting_name is None:
+            return False
+
+        return getattr(settings, setting_name, False)
+
+    @classmethod
+    def require(cls, flag_name):
+        """Decorator to check feature at the beginning of endpoint methods."""
+
+        if flag_name not in cls.FLAGS:
+            raise ValueError(f"Unknown feature flag: {flag_name}")
+
+        def decorator(view_func):
+            @wraps(view_func)
+            def wrapper(self, request, *args, **kwargs):
+                if not cls.flag_is_active(flag_name):
+                    raise Http404
+                return view_func(self, request, *args, **kwargs)
+
+            return wrapper
+
+        return decorator