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>

crates/colony/Cargo.toml

@@ -13,4 +13,5 @@ tokio = { workspace = true }
 sqlx = { workspace = true }
 axum = { workspace = true }
 tower-http = { workspace = true }
+futures-util = { workspace = true }
 thiserror = { workspace = true }