refactor: federation uses persistent WS instead of HTTP polling

- Server-to-server communication via WebSocket at /v1/federation/ws
- Auth as first WS frame (shared secret), presence + forwards over same connection
- Auto-reconnect every 3s on disconnect, instant presence push on connect
- Replaces HTTP REST polling (no more 5s intervals, lower latency)
- Removed dead HMAC helpers (auth is now direct secret comparison over WS)
- Simplified ARCHITECTURE.md mermaid diagrams for Gitea rendering

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

warzone/Cargo.toml

@@ -78,5 +78,8 @@ base64 = "0.22"
 # UUID
 uuid = { version = "1", features = ["v4", "serde"] }
 
+# WebSocket client
+tokio-tungstenite = { version = "0.21", features = ["native-tls"] }
+
 # Zero secrets in memory
 zeroize = { version = "1", features = ["derive"] }