feat: voice channel user limit (P5-10) + call join/leave sounds (P5-16)

P5-10 Voice Channel User Limit:
- New StateEvent.LotusVoiceLimit (io.lotus.voice_limit) with { max_users }
- RoomVoiceLimit admin control in Room Settings > General > Voice
  (power-level gated via permissions.stateEvent)
- CallPrescreen reads the limit reactively and disables Join with a
  'Channel Full (N/N)' message at capacity; existing members can rejoin

P5-16 Custom Join/Leave Sound Effects:
- useCallJoinLeaveSounds hook wired into CallUtils; detects participant
  join/leave via MatrixRTCSession membership changes (sender|deviceId),
  filters out self, only fires while joined
- Sounds synthesized in-browser with Web Audio (callSounds.ts) - no
  assets bundled; styles Off/Chime/Soft/Retro
- 'Join & Leave Sounds' selector in Settings > Calls (previews on change)

Docs: LOTUS_FEATURES.md, README.md, LOTUS_TODO.md (P5-10/P5-16 marked done)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/types/matrix/room.ts

@@ -40,6 +40,7 @@ export enum StateEvent {
 
   PoniesRoomEmotes = 'im.ponies.room_emotes',
   PowerLevelTags = 'in.cinny.room.power_level_tags',
+  LotusVoiceLimit = 'io.lotus.voice_limit',
 }
 
 export enum MessageEvent {