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/src/main.rs

@@ -49,12 +49,12 @@ async fn main() -> anyhow::Result<()> {
         state.federation = Some(handle);
     }
 
-    // Spawn federation presence sync if enabled
-    if let Some(ref federation) = state.federation {
-        let handle = federation.clone();
-        let connections = state.connections.clone();
+    // Spawn federation outgoing WS connection if enabled
+    if let Some(ref fed) = state.federation {
+        let handle = fed.clone();
+        let fed_state = state.clone();
         tokio::spawn(async move {
-            federation::presence_sync_loop(handle, connections).await;
+            federation::outgoing_ws_loop(handle, fed_state).await;
         });
     }