feat(rtc): Add event_types filtering to FfiQueue.subscribe() to reduce memory allocations

[Hormold]

Summary

Fix for #563
This PR adds an optional event_types parameter to FfiQueue.subscribe() that allows subscribers to filter events by type before call_soon_threadsafe() is called, preventing unnecessary object allocation.

Problem

FfiQueue.put() broadcasts ALL FFI events to ALL subscribers via call_soon_threadsafe(). Each call creates asyncio.Handle + contextvars.copy_context() objects. AudioStream and VideoStream filter events with wait_for(predicate), but objects are already allocated by then.

With N streams subscribed, this creates N × all_events objects, with 95%+ discarded after allocation.

Real-world impact observed in a 30-min meeting with 4 participants:

• 903,154 FFI events accumulated
• Memory grew from 312 MB to 1.29 GB (~12-15 MB/min)
• Event loop lag increased to 20+ seconds
• Transcription freezing due to event loop starvation

Solution

• Add optional event_types: Optional[Set[str]] parameter to FfiQueue.subscribe()
• Filter events by WhichOneof("message") before calling call_soon_threadsafe()
• AudioStream now subscribes with event_types={"audio_stream_event"}
• VideoStream now subscribes with event_types={"video_stream_event"}
• Full backwards compatibility: event_types=None receives all events (original behavior)

Results

With the patch applied:

• 95% reduction in object allocations for stream subscribers
• Memory stable at ~10 MB instead of growing to 1+ GB
• No event loop lag or transcription freezing

Testing

• Added unit tests for event filtering functionality
• Verified filtering behavior with multiple subscribers and mixed event types
• Tested in production environment with stable memory usage over 2+ hours

Files Changed

• livekit-rtc/livekit/rtc/_ffi_client.py - Add event_types filtering to FfiQueue
• livekit-rtc/livekit/rtc/audio_stream.py - Use filtered subscription
• livekit-rtc/livekit/rtc/video_stream.py - Use filtered subscription
• tests/rtc/test_ffi_queue.py - Unit tests for filtering