feat(livestream): add WebRTC ingest and playback

[zijiren233]

Summary

• add WHIP publishing and WHEP playback backed by the xiu StreamHub
• support external WHEP live proxy sources and expose WebRTC playback through the live/RTMP provider APIs
• relay RTP and WebRTC session lifecycle across cluster nodes with generation and lease fencing
• add WebRTC configuration, environment variables, CLI output, Helm values, OpenAPI definitions, and bilingual documentation
• keep frame and RTP relay liveness deadlines independent so a stalled path triggers relay recovery

Validation

• cargo fmt --all -- --check
• cargo clippy -p synctv-livestream --all-targets -- -D warnings
• cargo test -p synctv-livestream (239 passed, 20 ignored)
• git diff --check