fix: Use per-participant E2EE mode (no shared_key)

shared_key locks provider in shared-key mode, making set_key()
ineffective for per-participant decryption. Remove shared_key so
SDK initializes in per-participant mode. Also: failure_tolerance=-1
to prevent premature track closure on decrypt failures,
ratchet_window_size=16 to match Element Call.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

voice.py

@@ -69,16 +69,17 @@ def _generate_lk_jwt(room_id, user_id, device_id):
 KDF_HKDF = 1
 
 
-def _build_e2ee_options(shared_key: bytes) -> rtc.E2EEOptions:
+def _build_e2ee_options() -> rtc.E2EEOptions:
     """Build HKDF E2EE options matching Element Call's key derivation.
 
-    Element Call uses: ratchetWindowSize=10, keyringSize=256, salt="LKFrameEncryptionKey"
+    No shared_key — initializes in per-participant mode so set_key() works.
+    Element Call uses: ratchetWindowSize=16, keyringSize=256, salt="LKFrameEncryptionKey"
     """
     key_opts = rtc.KeyProviderOptions(
-        shared_key=shared_key,
-        ratchet_window_size=10,
+        shared_key=None,
+        ratchet_window_size=16,
         ratchet_salt=b"LKFrameEncryptionKey",
-        failure_tolerance=10,
+        failure_tolerance=-1,
         key_ring_size=256,
         key_derivation_function=KDF_HKDF,
     )
@@ -209,9 +210,9 @@ class VoiceSession:
                         break
                     await asyncio.sleep(0.1)
 
-            # Connect with E2EE enabled using bot's key as initial shared_key
-            # (required to initialize encryption framework, we override per-participant after)
-            e2ee_opts = _build_e2ee_options(self._bot_key)
+            # Connect with E2EE in per-participant mode (no shared_key)
+            # so set_key() calls work correctly for both directions
+            e2ee_opts = _build_e2ee_options()
             room_opts = rtc.RoomOptions(e2ee=e2ee_opts)
             self.lk_room = rtc.Room()