matrix-synapse: route MatrixRTC transport discovery to workers

Current Synapse registers the MatrixRTC transport discovery endpoint on
client-serving workers when MSC4143 is enabled, but MDAD does not model
that path in either its client-reader bucket or its broader generic-
worker endpoint list.

Add the unstable MatrixRTC transport discovery route so MDAD's worker
routing matches the current upstream worker surface for this endpoint.
This is a small, isolated routing addition for a simple authenticated
GET endpoint.

Refs:
- b99a58719b/synapse/rest/client/matrixrtc.py (L30-L52)
- b99a58719b/synapse/rest/__init__.py (L81-L129)
- b99a58719b/synapse/rest/__init__.py (L179-L197)

roles/custom/matrix-synapse/vars/main.yml

@@ -31,7 +31,7 @@ matrix_synapse_workers_sync_worker_client_server_endpoints:
 
 # Client reader workers handle generic client-server endpoints that don't contain a roomid or sync
 matrix_synapse_workers_client_reader_client_server_endpoints:
-  - ^/_matrix/client/(api/v1|r0|v3|unstable)/(room_keys/|keys/(query|changes|claim|room_keys/)|login|register(/available|/m.login.registration_token/validity|)|password_policy|profile|rooms/.*/(joined_members|context/.*|members|state|hierarchy|relations/|event/|aliases|timestamp_to_event|redact|send|state/|(join|invite|leave|ban|unban|kick))|createRoom|publicRooms|account/(3pid|whoami|deactivate)|versions|voip/turnServer|joined_rooms|search|user/.*/filter(/|$)|directory/room/.*|capabilities)
+  - ^/_matrix/client/(api/v1|r0|v3|unstable)/(room_keys/|keys/(query|changes|claim|room_keys/)|login|register(/available|/m.login.registration_token/validity|)|password_policy|profile|rooms/.*/(joined_members|context/.*|members|state|hierarchy|relations/|event/|aliases|timestamp_to_event|redact|send|state/|(join|invite|leave|ban|unban|kick))|createRoom|publicRooms|account/(3pid|whoami|deactivate)|versions|voip/turnServer|joined_rooms|search|user/.*/filter(/|$)|directory/room/.*|capabilities)|^/_matrix/client/unstable/org.matrix.msc4143/rtc/transports$
 
 # Federation reader workers handle generic federation endpoints that don't contain a roomid
 matrix_synapse_workers_federation_reader_federation_endpoints:
@@ -201,6 +201,7 @@ matrix_synapse_workers_generic_worker_endpoints:
   - ^/_matrix/client/v1/rooms/.*/threads$
   - ^/_matrix/client/unstable/im.nheko.summary/rooms/.*/summary$
   - ^/_matrix/client/unstable/im.nheko.summary/summary/.*$
+  - ^/_matrix/client/unstable/org.matrix.msc4143/rtc/transports$
   - ^/_matrix/client/(r0|v3|unstable)/account/3pid$
   - ^/_matrix/client/(r0|v3|unstable)/account/whoami$
   - ^/_matrix/client/(r0|v3|unstable)/account/deactivate$