fix: make call join/leave sounds audible to all participants + server-side hard voice limit docs

Sounds (P5-16): browsers block the Web Audio context until a user gesture
starts it, so join/leave sounds — which fire later with no gesture — were
silent. unlockCallSounds() now primes/resumes the shared AudioContext inside
the Join click (centralized in useCallStart so every join path is covered),
making the per-client sounds reliably audible to everyone in the call.

Voice limit (P5-10): the limit is now a hard, cross-client server-side cap
enforced by the voice-limit-guard sidecar (matrix repo) that fronts
lk-jwt-service and refuses LiveKit tokens when a room is full. Updated
LOTUS_FEATURES.md / README.md / LOTUS_TODO.md to reflect that the client
'Channel Full' check is UX only and the server is authoritative.

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

src/app/utils/callSounds.ts

@@ -12,6 +12,17 @@ const getAudioContext = (): AudioContext | undefined => {
   }
 };
 
+/**
+ * Create and resume the shared AudioContext from within a user gesture
+ * (e.g. clicking "Join"). Browsers block AudioContext playback until it has
+ * been started by a gesture, so join/leave sounds — which fire later without
+ * any gesture — would otherwise be silent. Call this on call entry so every
+ * participant's later membership-change sounds are actually audible.
+ */
+export const unlockCallSounds = (): void => {
+  getAudioContext();
+};
+
 type Note = {
   freq: number;
   /** Offset from now, in seconds */