feat: add WebRTC streaming via str0m + portal session persistence

- Add src/webrtc.rs: HTTP signaling server + str0m Sans-IO WebRTC transport
  with H.264 Annex-B → RTP packetization and key-frame request handling
- avhw: introduce FrameOutput enum (Muxer | Channel) so SwEncState can
  output to either MP4 muxer or crossbeam channel for WebRTC
- cap_portal: support portal session restore tokens (PersistMode::ExplicitlyRevoked)
  to skip re-authorization dialog; add --no-persist flag to force fresh dialog
- args: make --output optional when --port is used for WebRTC mode
- state_portal: integrate WebRTC pipeline (encoder channel → RTP forwarding)
  with shorter GOP for WebRTC (fps/2, min 10)
- main: redirect tracing to stderr; validate --output or --port required
- Add dependencies: str0m 0.20, serde_json 1, dirs 6

Cargo.toml

@@ -25,3 +25,6 @@ tokio = { version = "1", features = ["rt"] }
 pipewire = { version = "0.9", features = ["v0_3_45"] }
 libspa = "0.9"
 crossbeam-channel = "0.5"
+str0m = "0.20"
+serde_json = "1"
+dirs = "6"