S5: WebSocket real-time — per-channel broadcast, auto-reconnect

Backend:
- AppState with per-channel broadcast::Sender map
- WS handler: auth via first message, keepalive pings, broadcast forwarding
- post_message broadcasts WsEvent::Message to all subscribers

Frontend:
- useChannelSocket hook: connects, auths, appends messages, auto-reconnects
- Removed 3s polling — WebSocket is primary, initial load via REST
- Deduplication on WS messages (sender also fetches after post)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -12,4 +12,5 @@ sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite", "chrono", "uuid
 axum = { version = "0.8", features = ["ws"] }
 tower-http = { version = "0.6", features = ["cors", "fs"] }
 ts-rs = { version = "10", features = ["serde-json-impl", "uuid-impl", "chrono-impl"] }
+futures-util = "0.3"
 thiserror = "2"