⚡️(backend) replace blocking Redis KEYS with cursor-based SCAN

`cache.keys()` runs Redis `KEYS`, a full-keyspace scan on Redis's
single thread that blocks everything else, including session reads
in the same cache. Its cost scales with total keys, not matches,
and some managed providers disable `KEYS` entirely.

The trusted-lobby feature made this urgent: the waiting-list
endpoint scanned on every poll, and its polling audience grows from
a few admins to potentially every authenticated participant.

Switch to cursor-based `SCAN` via two `core.utils` helpers, deleting
in bounded batches so cleanup of a large room cannot block either.
A single seam also lets us forbid raw `cache.keys()` going forward.

`SCAN` still iterates the keyspace incrementally on the polled
path. If monitoring flags it, the follow-up is a per-room set
index — out of scope here since it changes the lobby storage model.

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

@@ -24,6 +24,7 @@ from core.services.lobby import (
     LobbyParticipantStatus,
     LobbyService,
 )
+from core.services.presence import CACHE_SCAN_ITERSIZE
 from core.utils import NotificationError
 
 pytestmark = pytest.mark.django_db
@@ -578,14 +579,14 @@ def test_get_participant_parsing_error(
 @mock.patch("core.services.lobby.cache")
 def test_list_waiting_participants_empty(mock_cache, lobby_service):
     """Test listing waiting participants when none exist."""
-    mock_cache.keys.return_value = []
+    mock_cache.iter_keys.return_value = []
 
     room = RoomFactory(access_level=RoomAccessLevel.RESTRICTED)
     result = lobby_service.list_waiting_participants(room.id)
 
     assert result == []
     pattern = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_*"
-    mock_cache.keys.assert_called_once_with(pattern)
+    mock_cache.iter_keys.assert_called_once_with(pattern, itersize=CACHE_SCAN_ITERSIZE)
     mock_cache.get_many.assert_not_called()
 
 
@@ -594,7 +595,7 @@ def test_list_waiting_participants(mock_cache, lobby_service, participant_dict):
     """Test listing waiting participants with valid data."""
     room = RoomFactory(access_level=RoomAccessLevel.RESTRICTED)
     cache_key = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_participant1"
-    mock_cache.keys.return_value = [cache_key]
+    mock_cache.iter_keys.return_value = [cache_key]
     mock_cache.get_many.return_value = {cache_key: participant_dict}
 
     result = lobby_service.list_waiting_participants(room.id)
@@ -603,7 +604,7 @@ def test_list_waiting_participants(mock_cache, lobby_service, participant_dict):
     assert result[0]["status"] == "waiting"
     assert result[0]["username"] == "test-username"
     pattern = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_*"
-    mock_cache.keys.assert_called_once_with(pattern)
+    mock_cache.iter_keys.assert_called_once_with(pattern, itersize=CACHE_SCAN_ITERSIZE)
     mock_cache.get_many.assert_called_once_with([cache_key])
 
 
@@ -628,7 +629,7 @@ def test_list_waiting_participants_multiple(mock_cache, lobby_service):
         "color": "#654321",
     }
 
-    mock_cache.keys.return_value = [cache_key1, cache_key2]
+    mock_cache.iter_keys.return_value = [cache_key1, cache_key2]
     mock_cache.get_many.return_value = {
         cache_key1: participant1,
         cache_key2: participant2,
@@ -646,7 +647,7 @@ def test_list_waiting_participants_multiple(mock_cache, lobby_service):
     assert all(p["status"] == "waiting" for p in result)
 
     pattern = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_*"
-    mock_cache.keys.assert_called_once_with(pattern)
+    mock_cache.iter_keys.assert_called_once_with(pattern, itersize=CACHE_SCAN_ITERSIZE)
     mock_cache.get_many.assert_called_once_with([cache_key1, cache_key2])
 
 
@@ -655,7 +656,7 @@ def test_list_waiting_participants_corrupted_data(mock_cache, lobby_service):
     """Test listing waiting participants with corrupted data."""
     room = RoomFactory(access_level=RoomAccessLevel.RESTRICTED)
     cache_key = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_participant1"
-    mock_cache.keys.return_value = [cache_key]
+    mock_cache.iter_keys.return_value = [cache_key]
     mock_cache.get_many.return_value = {cache_key: {"invalid": "data"}}
 
     result = lobby_service.list_waiting_participants(room.id)
@@ -680,7 +681,7 @@ def test_list_waiting_participants_partially_corrupted(mock_cache, lobby_service
 
     corrupted_participant = {"invalid": "data"}
 
-    mock_cache.keys.return_value = [cache_key1, cache_key2]
+    mock_cache.iter_keys.return_value = [cache_key1, cache_key2]
     mock_cache.get_many.return_value = {
         cache_key1: corrupted_participant,
         cache_key2: valid_participant,
@@ -699,7 +700,7 @@ def test_list_waiting_participants_partially_corrupted(mock_cache, lobby_service
 
     # Verify both cache keys were queried
     pattern = f"{settings.LOBBY_KEY_PREFIX}_{room.id!s}_*"
-    mock_cache.keys.assert_called_once_with(pattern)
+    mock_cache.iter_keys.assert_called_once_with(pattern, itersize=CACHE_SCAN_ITERSIZE)
     mock_cache.get_many.assert_called_once_with([cache_key1, cache_key2])
 
 
@@ -723,7 +724,7 @@ def test_list_waiting_participants_non_waiting(mock_cache, lobby_service):
         "color": "#654321",
     }
 
-    mock_cache.keys.return_value = [cache_key1, cache_key2]
+    mock_cache.iter_keys.return_value = [cache_key1, cache_key2]
     mock_cache.get_many.return_value = {
         cache_key1: participant1,
         cache_key2: participant2,

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

@@ -88,3 +88,21 @@ def test_presence_clear_and_clear_room():
     presence.clear_room(room_id)
     assert presence.is_marked_present(room_id, "b") is False
     assert presence.is_marked_present(other_room, "a") is True
+
+
+def test_presence_clear_room_scans_in_pages():
+    """clear_room removes every match, even across several SCAN pages,
+    and only within the room."""
+    room_id, other_room = str(uuid4()), str(uuid4())
+    presence = PresenceCache()
+    for i in range(7):
+        presence.mark_present(room_id, f"user-{i}")
+    presence.mark_present(other_room, "user-0")
+
+    # An itersize smaller than the match count forces delete_pattern to
+    # page through several SCAN cursors rather than finish in one pass.
+    with mock.patch("core.utils.CACHE_SCAN_ITERSIZE", 3):
+        presence.clear_room(room_id)
+
+    assert all(not presence.is_marked_present(room_id, f"user-{i}") for i in range(7))
+    assert presence.is_marked_present(other_room, "user-0") is True