✨(backend) add lobby cache clearing method for meeting conclusion

Implement new lobby service method to clear all participant entries from cache.

Lays foundation for upcoming feature where participant permissions reset when
meetings end. Currently introduces only the cache clearing functionality;
event handling for meeting conclusion will be implemented in future commits

src/backend/core/services/lobby_service.py

@@ -356,3 +356,14 @@ class LobbyService:
             raise LobbyNotificationError("Failed to notify room participants") from e
         finally:
             await lkapi.aclose()
+
+    def clear_room_cache(self, room_id: UUID) -> None:
+        """Clear all participant entries from the cache for a specific room."""
+
+        pattern = self._get_cache_key(room_id, "*")
+        keys = cache.keys(pattern)
+
+        if not keys:
+            return
+
+        cache.delete_many(keys)

src/backend/core/tests/services/test_lobby_service.py

@@ -6,9 +6,11 @@ Test lobby service.
 # ruff: noqa: PLR0913
 
 import json
+import uuid
 from unittest import mock
 
 from django.conf import settings
+from django.core.cache import cache
 from django.http import HttpResponse
 
 import pytest
@@ -832,3 +834,60 @@ def test_notify_participants_error(mock_livekit_api, lobby_service):
 
     # Verify aclose was still called after the exception
     mock_api_instance.aclose.assert_called_once()
+
+
+def test_clear_room_cache(settings, lobby_service):
+    """Test clearing room cache actually removes entries from cache."""
+
+    settings.LOBBY_KEY_PREFIX = "test-lobby"
+    settings.LOBBY_WAITING_TIMEOUT = 10000
+    settings.LOBBY_ACCEPTED_TIMEOUT = 10000
+    settings.LOBBY_DENIED_TIMEOUT = 10000
+
+    room_id = uuid.uuid4()
+
+    cache.set(
+        f"test-lobby_{room_id!s}_participant1",
+        LobbyParticipant(
+            status=LobbyParticipantStatus.WAITING,
+            username="participant1",
+            id="participant1",
+            color="#123456",
+        ),
+        timeout=settings.LOBBY_WAITING_TIMEOUT,
+    )
+    cache.set(
+        f"test-lobby_{room_id!s}_participant2",
+        LobbyParticipant(
+            status=LobbyParticipantStatus.ACCEPTED,
+            username="participant2",
+            id="participant2",
+            color="#123456",
+        ),
+        timeout=settings.LOBBY_ACCEPTED_TIMEOUT,
+    )
+    cache.set(
+        f"test-lobby_{room_id!s}_participant3",
+        LobbyParticipant(
+            status=LobbyParticipantStatus.DENIED,
+            username="participant3",
+            id="participant3",
+            color="#123456",
+        ),
+        timeout=settings.LOBBY_DENIED_TIMEOUT,
+    )
+
+    lobby_service.clear_room_cache(room_id)
+
+    assert cache.keys(f"test-lobby_{room_id!s}_*") == []
+
+
+def test_clear_room_empty(settings, lobby_service):
+    """Test clearing room cache when it's already empty."""
+
+    settings.LOBBY_KEY_PREFIX = "test-lobby"
+    room_id = uuid.uuid4()
+
+    assert cache.keys(f"test-lobby_{room_id!s}_*") == []
+    lobby_service.clear_room_cache(room_id)
+    assert cache.keys(f"test-lobby_{room_id!s}_*") == []