Add dedicated \`GET /_matrix/client/v1/rtc/transports\` and \`GET /_matrix/client/unstable/org.matrix.msc4143/rtc/transports\` endpoints for MatrixRTC focus discovery (MSC4143), replacing the deprecated well-known approach.
Move RTC foci configuration from \`[global.well_known]\` into a new \`[global.matrix_rtc]\` config section with a \`foci\` field. Remove \`rtc_foci\` from the \`.well-known/matrix/client\` response. Update LiveKit setup documentation accordingly.
Closes#1431