feat: 社区准则改为数据库存储

- AuthController 注入 SiteSettings 依赖，RegisterPage 优先从 DB 读取准则
- DB 中 site.guidelines 为空时 fallback 到静态 guidelines.html 文件
- 管理后台站点设置页新增「内容管理」分区，textarea 编辑社区准则
- 添加准则编辑区垂直布局样式

internal/router/deps_core.go

@@ -78,7 +78,7 @@ func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSet
 	sessionMgr := session.NewManager(sessionStore, userRepo, cfg)
 	authService := service.NewAuthService(userRepo, sessionMgr, cfg, siteSettings)
 	rateLimiter := middleware.NewRateLimiter()
-	authCtrl := controller.NewAuthController(authService, sessionMgr, rateLimiter, cfg)
+	authCtrl := controller.NewAuthController(authService, sessionMgr, rateLimiter, cfg, siteSettings)
 	avatarSvc := service.NewAvatarService(userRepo)
 	authMdw := middleware.NewAuthMiddleware(cfg, sessionMgr)
 	return userRepo, sessionMgr, authService, rateLimiter, authCtrl, avatarSvc, authMdw