fix(auth): clear stale Supabase auth cookies on invalid session

When getUser() rejects a present session cookie with user_not_found /
session_not_found / bad_jwt (e.g. a deleted account or a cookie that
outlived its session), the middleware now clears the sb-*-auth-token
cookies and redirects to /login instead of leaving the browser wedged
in a logged-out page that still renders a Sign out button.

middleware.ts

@@ -45,6 +45,7 @@ export async function middleware(request: NextRequest) {
 
   const {
     data: { user },
+    error: userError,
   } = await supabase.auth.getUser();
 
   const path = request.nextUrl.pathname;
@@ -67,6 +68,52 @@ export async function middleware(request: NextRequest) {
     return withCookies(NextResponse.redirect(url));
   };
 
+  // If a session cookie is present but the auth server rejects it because the
+  // underlying user or session no longer exists (a deleted account, or a stale
+  // cookie that outlived its session), proactively clear the Supabase auth
+  // cookies. Otherwise the browser keeps a token whose `sub` is gone, every
+  // `getUser()` 403s with `user_not_found`, and the UI gets wedged in a
+  // confusing "logged-out page that still shows a Sign out button" state.
+  const INVALID_SESSION_CODES = new Set([
+    'user_not_found',
+    'session_not_found',
+    'session_expired',
+    'refresh_token_not_found',
+    'bad_jwt',
+  ]);
+  const isAuthCookie = (name: string): boolean =>
+    name.startsWith('sb-') && name.includes('-auth-token');
+  if (
+    !user &&
+    userError &&
+    INVALID_SESSION_CODES.has(userError.code ?? '') &&
+    request.cookies.getAll().some((c) => isAuthCookie(c.name))
+  ) {
+    const clearStale = (res: NextResponse): NextResponse => {
+      request.cookies.getAll().forEach((c) => {
+        if (isAuthCookie(c.name)) {
+          res.cookies.set(c.name, '', { maxAge: 0, path: '/' });
+        }
+      });
+      res.headers.set('Cache-Control', 'no-store');
+      return res;
+    };
+    if (path.startsWith('/api/')) {
+      return clearStale(
+        NextResponse.json({ error: 'unauthorized' }, { status: 401 }),
+      );
+    }
+    if (!isMfaAllowlisted(path)) {
+      const url = request.nextUrl.clone();
+      url.pathname = '/login';
+      url.search = '';
+      return clearStale(NextResponse.redirect(url));
+    }
+    // Allowlisted pages (e.g. /login itself) render fine logged-out — just drop
+    // the dead cookies so the nav is consistent on the next request.
+    return clearStale(NextResponse.next({ request }));
+  }
+
   // Defense-in-depth: guard the admin surface here in addition to the
   // per-route requireAdmin()/requireAdminApi() checks.
   if (path.startsWith('/admin') || path.startsWith('/api/admin')) {