feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

backend/middleware/context.middleware.ts

@@ -0,0 +1,18 @@
+/**
+ * Context Middleware
+ * Extracts X-DAO-ID header and attaches to request context
+ */
+
+import type { Request, Response, NextFunction } from 'express';
+
+export function contextMiddleware(req: Request, res: Response, next: NextFunction): void {
+  const daoId = req.headers['x-dao-id'] as string | undefined;
+  
+  if (daoId) {
+    (req as any).daoId = daoId;
+  }
+  
+  next();
+}
+
+