feat: 管理首页帖子总数统计（总数/已发布/待审核）

- PostRepo 新增 CountPostsByStatus 和 CountPending 方法
- Service 层定义 postAdminStatStore 接口 (ISP)
- AdminService 注入 postRepo，新增 PostStats 结构体和 GetPostStats 方法
- AdminController.Dashboard 调用 GetPostStats 并传参到模板
- Dashboard 帖子卡片显示总数 + 已发布 + 待审核（橙色徽章）
- DI 层将 AdminController 创建从 buildDepsCore 移至 buildDeps，以便注入 postRepo

internal/router/deps_core.go

@@ -34,7 +34,7 @@ type dependencies struct {
 func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings, redisClient *redis.Client) (
 	*repository.UserRepo, *session.Manager, *service.AuthService,
 	*middleware.RateLimiter, *controller.AuthController,
-	*service.AvatarService, *middleware.AuthMiddleware, *adminCtrl.AdminController,
+	*service.AvatarService, *middleware.AuthMiddleware,
 ) {
 	userRepo := repository.NewUserRepo(db)
 
@@ -65,6 +65,5 @@ func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSet
 	authCtrl := controller.NewAuthController(authService, sessionMgr, rateLimiter, cfg)
 	avatarSvc := service.NewAvatarService(userRepo)
 	authMdw := middleware.NewAuthMiddleware(cfg, sessionMgr)
-	adminController := adminCtrl.NewAdminController(service.NewAdminService(userRepo, sessionMgr))
-	return userRepo, sessionMgr, authService, rateLimiter, authCtrl, avatarSvc, authMdw, adminController
+	return userRepo, sessionMgr, authService, rateLimiter, authCtrl, avatarSvc, authMdw
 }