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/auth.middleware.ts

@@ -0,0 +1,39 @@
+/**
+ * Auth Middleware
+ * Validates Bearer token
+ * MVP: Simple validation, replace with proper JWT validation later
+ */
+
+import type { Request, Response, NextFunction } from 'express';
+
+export function authMiddleware(req: Request, res: Response, next: NextFunction): void {
+  const authHeader = req.headers.authorization;
+  
+  if (!authHeader || !authHeader.startsWith('Bearer ')) {
+    res.status(401).json({
+      error: 'UNAUTHORIZED',
+      message: 'Missing or invalid Authorization header',
+    });
+    return;
+  }
+  
+  const token = authHeader.substring(7);
+  
+  // MVP: Simple validation (just check token exists)
+  // TODO: Validate JWT token properly
+  if (!token) {
+    res.status(401).json({
+      error: 'UNAUTHORIZED',
+      message: 'Invalid token',
+    });
+    return;
+  }
+  
+  // Attach user ID to request (MVP: extract from token)
+  // TODO: Decode JWT and extract userId
+  (req as any).userId = 'user_stub'; // Replace with actual user ID from token
+  
+  next();
+}
+
+