The RTC transports API was enabled based on matrix_rtc_enabled, while the
transports it serves were populated based on matrix_livekit_jwt_service_enabled.
Enabling the LiveKit JWT service on its own (say, when pointing it at an
externally hosted LiveKit server) therefore configured a transport but left
the API which announces it switched off.

Gating the feature on matrix_synapse_matrix_rtc_transports instead keeps the
API and its payload in sync, and makes the empty-transports case that the
variable's own documentation warns about unreachable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>