feat(admin): 站点信息 + 注册开关可后台设置，模板动态渲染

新增 4 项可配置站点设置（DB持久化+内存缓存，即时生效）：
- site.brand          品牌名（页面标题/页脚/管理面板，默认 MetaLab）
- site.framework      框架名（页脚标识，默认 MetaZone）
- site.copyright_start 版权起始年（页脚，默认 2024）
- registration.enabled 注册开关（关闭后禁止新用户注册，默认 true）

实现方式：
- InjectSiteInfo 中间件：全局注入 Brand/Framework/CopyrightStart 到 gin.Context
- BuildPageData 读取 context，所有 SSR 页面自动获取站点信息
- AuthService.Register 最先检查 IsRegistrationEnabled()
- 管理后台新增「基本设置」+「注册设置」区块，文本输入手动保存 + Toggle 即时生效

internal/router/deps_core.go

@@ -22,14 +22,14 @@ type dependencies struct {
 	tokenCtrl       *controller.AuthController
 }
 
-func buildDepsCore(db *gorm.DB, cfg *config.Config) (
+func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings) (
 	*repository.UserRepo, *service.TokenService, *service.AuthService,
 	*middleware.RateLimiter, *controller.AuthController,
 	*service.AvatarService, *middleware.AuthMiddleware, *adminCtrl.AdminController,
 ) {
 	userRepo := repository.NewUserRepo(db)
 	tokenSvc := service.NewTokenService(cfg, userRepo)
-	authService := service.NewAuthService(userRepo, tokenSvc, cfg)
+	authService := service.NewAuthService(userRepo, tokenSvc, cfg, siteSettings)
 	rateLimiter := middleware.NewRateLimiter()
 	authCtrl := controller.NewAuthController(authService, tokenSvc, rateLimiter, cfg)
 	avatarSvc := service.NewAvatarService(userRepo)

internal/router/deps_extra.go

@@ -11,7 +11,7 @@ import (
 )
 
 func buildDeps(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings) *dependencies {
-	userRepo, _, authService, _, authCtrl, avatarSvc, authMdw, adminController := buildDepsCore(db, cfg)
+	userRepo, _, authService, _, authCtrl, avatarSvc, authMdw, adminController := buildDepsCore(db, cfg, siteSettings)
 
 	auditRepo := repository.NewAuditRepo(db)
 	notificationRepo := repository.NewNotificationRepo(db)

internal/router/router.go

@@ -11,6 +11,7 @@ import (
 // Setup 注册所有路由
 func Setup(r *gin.Engine, db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings) {
 	r.Use(middleware.SecurityHeaders())
+	r.Use(middleware.InjectSiteInfo(siteSettings))
 
 	deps := buildDeps(db, cfg, siteSettings)