fix: prevent presence race condition during WebRTC handshake

Add /joining and /joined endpoints to track user join intent before
WebRTC handshake completes. This prevents meetings from being
deactivated while users are still connecting.

- Add pending_joins Redis module with 30s TTL
- Add /joining endpoint (called before WebRTC handshake)
- Add /joined endpoint (called after connection established)
- Check for pending joins before deactivating meetings in worker
- Frontend integration with connectionId per browser tab

server/tests/test_pending_joins.py

@@ -0,0 +1,153 @@
+"""Tests for pending joins Redis helper functions.
+
+TDD tests for tracking join intent to prevent race conditions during
+WebRTC handshake when users join meetings.
+"""
+
+from unittest.mock import AsyncMock
+
+import pytest
+
+from reflector.presence.pending_joins import (
+    PENDING_JOIN_PREFIX,
+    PENDING_JOIN_TTL,
+    create_pending_join,
+    delete_pending_join,
+    has_pending_joins,
+)
+
+
+@pytest.fixture
+def mock_redis():
+    """Mock async Redis client."""
+    redis = AsyncMock()
+    redis.setex = AsyncMock()
+    redis.delete = AsyncMock()
+    redis.scan = AsyncMock(return_value=(0, []))
+    return redis
+
+
+@pytest.mark.asyncio
+async def test_create_pending_join_sets_key_with_ttl(mock_redis):
+    """Test that create_pending_join stores key with correct TTL."""
+    meeting_id = "meeting-123"
+    user_id = "user-456"
+
+    await create_pending_join(mock_redis, meeting_id, user_id)
+
+    expected_key = f"{PENDING_JOIN_PREFIX}:{meeting_id}:{user_id}"
+    mock_redis.setex.assert_called_once()
+    call_args = mock_redis.setex.call_args
+    assert call_args[0][0] == expected_key
+    assert call_args[0][1] == PENDING_JOIN_TTL
+    # Value should be a timestamp string
+    assert call_args[0][2] is not None
+
+
+@pytest.mark.asyncio
+async def test_delete_pending_join_removes_key(mock_redis):
+    """Test that delete_pending_join removes the key."""
+    meeting_id = "meeting-123"
+    user_id = "user-456"
+
+    await delete_pending_join(mock_redis, meeting_id, user_id)
+
+    expected_key = f"{PENDING_JOIN_PREFIX}:{meeting_id}:{user_id}"
+    mock_redis.delete.assert_called_once_with(expected_key)
+
+
+@pytest.mark.asyncio
+async def test_has_pending_joins_returns_false_when_no_keys(mock_redis):
+    """Test has_pending_joins returns False when no matching keys."""
+    mock_redis.scan.return_value = (0, [])
+
+    result = await has_pending_joins(mock_redis, "meeting-123")
+
+    assert result is False
+    mock_redis.scan.assert_called_once()
+    call_kwargs = mock_redis.scan.call_args.kwargs
+    assert call_kwargs["match"] == f"{PENDING_JOIN_PREFIX}:meeting-123:*"
+
+
+@pytest.mark.asyncio
+async def test_has_pending_joins_returns_true_when_keys_exist(mock_redis):
+    """Test has_pending_joins returns True when matching keys found."""
+    mock_redis.scan.return_value = (0, [b"pending_join:meeting-123:user-1"])
+
+    result = await has_pending_joins(mock_redis, "meeting-123")
+
+    assert result is True
+
+
+@pytest.mark.asyncio
+async def test_has_pending_joins_scans_with_correct_pattern(mock_redis):
+    """Test has_pending_joins uses correct scan pattern."""
+    meeting_id = "meeting-abc-def"
+    mock_redis.scan.return_value = (0, [])
+
+    await has_pending_joins(mock_redis, meeting_id)
+
+    expected_pattern = f"{PENDING_JOIN_PREFIX}:{meeting_id}:*"
+    mock_redis.scan.assert_called_once()
+    call_kwargs = mock_redis.scan.call_args.kwargs
+    assert call_kwargs["match"] == expected_pattern
+    assert call_kwargs["count"] == 100
+
+
+@pytest.mark.asyncio
+async def test_multiple_users_pending_joins(mock_redis):
+    """Test that multiple users can have pending joins for same meeting."""
+    meeting_id = "meeting-123"
+    # Simulate two pending joins
+    mock_redis.scan.return_value = (
+        0,
+        [b"pending_join:meeting-123:user-1", b"pending_join:meeting-123:user-2"],
+    )
+
+    result = await has_pending_joins(mock_redis, meeting_id)
+
+    assert result is True
+
+
+@pytest.mark.asyncio
+async def test_pending_join_ttl_value():
+    """Test that PENDING_JOIN_TTL has expected value."""
+    # 30 seconds should be enough for WebRTC handshake but not too long
+    assert PENDING_JOIN_TTL == 30
+
+
+@pytest.mark.asyncio
+async def test_pending_join_prefix_value():
+    """Test that PENDING_JOIN_PREFIX has expected value."""
+    assert PENDING_JOIN_PREFIX == "pending_join"
+
+
+@pytest.mark.asyncio
+async def test_has_pending_joins_multi_iteration_scan_no_keys(mock_redis):
+    """Test has_pending_joins iterates until cursor returns 0."""
+    # Simulate multi-iteration scan: cursor 100 -> cursor 50 -> cursor 0
+    mock_redis.scan.side_effect = [
+        (100, []),  # First iteration, no keys, continue
+        (50, []),  # Second iteration, no keys, continue
+        (0, []),  # Third iteration, cursor 0, done
+    ]
+
+    result = await has_pending_joins(mock_redis, "meeting-123")
+
+    assert result is False
+    assert mock_redis.scan.call_count == 3
+
+
+@pytest.mark.asyncio
+async def test_has_pending_joins_multi_iteration_finds_key_later(mock_redis):
+    """Test has_pending_joins finds key on second iteration."""
+    # Simulate finding key on second scan iteration
+    mock_redis.scan.side_effect = [
+        (100, []),  # First iteration, no keys
+        (0, [b"pending_join:meeting-123:user-1"]),  # Second iteration, found key
+    ]
+
+    result = await has_pending_joins(mock_redis, "meeting-123")
+
+    assert result is True
+    assert mock_redis.scan.call_count == 2