fix: 修复 admin-auth 测试的 TypeScript 类型错误

将 mockDb 的类型从 Record<string, Mock> 改为显式的映射类型，
消除 CI 中 "possibly undefined" 的类型检查报错。

src/__tests__/services/admin/admin-auth.test.ts

@@ -9,7 +9,7 @@ vi.mock('bcryptjs', () => ({
 
 // Build the mock DB once, then restore individual mocks in beforeEach
 function buildMockDb() {
-  const db: Record<string, Mock> = {};
+  const db: { [K in 'select'|'from'|'where'|'limit'|'update'|'set']: Mock } = {} as typeof db;
   db.select = vi.fn().mockReturnValue(db);
   db.from = vi.fn().mockReturnValue(db);
   db.where = vi.fn().mockReturnValue(db);