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/crates/warzone-server/Cargo.toml

@@ -27,3 +27,4 @@ ed25519-dalek.workspace = true
 bincode.workspace = true
 sha2.workspace = true
 reqwest = { workspace = true, features = ["rustls-tls", "json"] }
+tokio-tungstenite.workspace = true