feat: Add login/logout audit logging (refs #10)

Backend:
- Add login event logging to getUserStatus() controller method
- Create POST /auth/track-logout endpoint for logout tracking

Frontend:
- Create useLogout hook that wraps Auth0 logout with audit tracking
- Update all logout locations to use the new hook (SettingsPage,
  Layout, MobileSettingsScreen, useDeletion)

Login events are logged when the frontend calls /auth/user-status after
Auth0 callback. Logout events are logged via fire-and-forget call to
/auth/track-logout before Auth0 logout.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/core/auth/useLogout.ts

@@ -0,0 +1,46 @@
+/**
+ * @ai-summary Custom logout hook with audit logging
+ * @ai-context Tracks logout event before Auth0 logout for audit trail
+ */
+
+import { useAuth0 } from '@auth0/auth0-react';
+import { useCallback } from 'react';
+
+/**
+ * Custom hook that wraps Auth0 logout with audit tracking.
+ * Calls /api/auth/track-logout before performing Auth0 logout.
+ * The audit call is fire-and-forget to ensure logout always completes.
+ */
+export const useLogout = () => {
+  const { logout: auth0Logout, getAccessTokenSilently } = useAuth0();
+
+  const logout = useCallback(async () => {
+    // Fire-and-forget audit call (don't block logout)
+    try {
+      const token = await getAccessTokenSilently({ cacheMode: 'on' as const });
+      if (token) {
+        // Use fetch directly to avoid axios interceptor issues during logout
+        fetch('/api/auth/track-logout', {
+          method: 'POST',
+          headers: {
+            'Authorization': `Bearer ${token}`,
+            'Content-Type': 'application/json',
+          },
+        }).catch(() => {
+          // Silently ignore errors - don't block logout
+        });
+      }
+    } catch {
+      // Token not available - proceed with logout anyway
+    }
+
+    // Perform Auth0 logout
+    auth0Logout({
+      logoutParams: {
+        returnTo: window.location.origin,
+      },
+    });
+  }, [auth0Logout, getAccessTokenSilently]);
+
+  return { logout };
+};