feat(distribution): 公告中心支持分站自定义

公告中心（前台公告列表/详情）此前完全没有站点概念，分站访客只能看到主站公告。
本次按「默认继承主站、站长可切仅本站」的口径支持分站自定义。

数据与口径：
- 新增 announcements.distribution_site_id（0=主站），前台按站点隔离
- 新增 distribution_sites.announcement_mode：inherit（默认，主站+本站）/ own_only（仅本站）

后端：
- 公告模块新增 ResolvePublicSiteScope：主站 [0]、inherit [0, siteID]、own_only [siteID]，
  站点编码无效/已停用回落主站；列表与详情都按该范围过滤，详情不再允许跨站按 ID 读取
- 管理端公告列表支持按站点筛选（site_id：不传=全部，0=主站，>0=分站）并带出站点名
- 分销模块新增本站公告 CRUD（distribution/announcement.go）：强制 distribution_site_id=本站，
  先过 ensureSiteAccess，跨站更新/删除/上下架一律 ErrAnnouncementNotFound
- UpdateSiteBranding 支持 announcement_mode 切换（非法值报错，读侧非法历史值回落 inherit）
- 新增权限 distribution:announcement_view|manage 并授予 super_admin/ops/finance，
  站长开通时继承 ops 角色即自动获得
- 路由：站长侧 /api/distribution/announcements*，平台侧 /api/admin/distribution/announcements*

前端：
- 分销后台新增「公告中心」页：来源开关（继承主站+本站 / 仅本站）、本站公告列表与
  新建/编辑/发布/下架/归档/恢复/删除，空态按来源模式给出引导
- 平台后台公告页新增「站点」列与筛选，新建公告可选择归属站点（主站或指定分站代发）
- 前台 PC/移动无需改动：请求已带 X-Distribution-Site-Code，后端按站点隔离，页面已有空态

测试：
- 公告模块：主站/继承/仅本站三种可见范围、跨站详情拦截、无效站点回落、管理端站点筛选
- 分销模块：本站公告 CRUD 与跨站防护、公告来源开关校验与非法值容错

backend/internal/router/router.go

@@ -641,6 +641,14 @@ func New(cfg config.Config, deps Dependencies, logger *zap.Logger) *gin.Engine {
 			distributionProtectedRoutes.GET("/sites/:id/analytics", distributionHandler.GetDashboardAnalytics)
 			distributionProtectedRoutes.GET("/received-listings", distributionHandler.ListReceivedListings)
 			distributionProtectedRoutes.PUT("/received-listings/:id/status", distributionHandler.UpdateReceivedListingStatus)
+			// 分站公告中心：站长自助维护本站公告（站点由 ?site_id= 解析并校验成员权限）
+			distributionProtectedRoutes.GET("/announcements", distributionHandler.ListSiteAnnouncements)
+			distributionProtectedRoutes.POST("/announcements", distributionHandler.CreateSiteAnnouncement)
+			distributionProtectedRoutes.PUT("/announcements/:id", distributionHandler.UpdateSiteAnnouncement)
+			distributionProtectedRoutes.POST("/announcements/:id/publish", distributionHandler.PublishSiteAnnouncement)
+			distributionProtectedRoutes.POST("/announcements/:id/archive", distributionHandler.ArchiveSiteAnnouncement)
+			distributionProtectedRoutes.POST("/announcements/:id/unarchive", distributionHandler.UnarchiveSiteAnnouncement)
+			distributionProtectedRoutes.DELETE("/announcements/:id", distributionHandler.DeleteSiteAnnouncement)
 			distributionProtectedRoutes.PUT("/accounts/:id/markup", distributionHandler.UpdateAccountMarkup)
 			distributionProtectedRoutes.POST("/accounts/batch-markup", distributionHandler.BatchUpdateAccountMarkup)
 			distributionProtectedRoutes.POST("/accounts/batch-display", distributionHandler.BatchUpdateAccountDisplay)
@@ -674,6 +682,13 @@ func New(cfg config.Config, deps Dependencies, logger *zap.Logger) *gin.Engine {
 			adminRoutes.GET("/distribution/sites/:id/analytics", requirePerm("distribution:site_view"), distributionHandler.GetDashboardAnalytics)
 			adminRoutes.GET("/distribution/received-listings", requirePerm("distribution:account_view"), distributionHandler.ListReceivedListings)
 			adminRoutes.PUT("/distribution/received-listings/:id/status", requirePerm("distribution:account_display"), distributionHandler.UpdateReceivedListingStatus)
+			adminRoutes.GET("/distribution/announcements", requirePerm("distribution:announcement_view"), distributionHandler.ListSiteAnnouncements)
+			adminRoutes.POST("/distribution/announcements", requirePerm("distribution:announcement_manage"), distributionHandler.CreateSiteAnnouncement)
+			adminRoutes.PUT("/distribution/announcements/:id", requirePerm("distribution:announcement_manage"), distributionHandler.UpdateSiteAnnouncement)
+			adminRoutes.POST("/distribution/announcements/:id/publish", requirePerm("distribution:announcement_manage"), distributionHandler.PublishSiteAnnouncement)
+			adminRoutes.POST("/distribution/announcements/:id/archive", requirePerm("distribution:announcement_manage"), distributionHandler.ArchiveSiteAnnouncement)
+			adminRoutes.POST("/distribution/announcements/:id/unarchive", requirePerm("distribution:announcement_manage"), distributionHandler.UnarchiveSiteAnnouncement)
+			adminRoutes.DELETE("/distribution/announcements/:id", requirePerm("distribution:announcement_manage"), distributionHandler.DeleteSiteAnnouncement)
 			adminRoutes.GET("/distribution/rules", requirePerm("distribution:rule_view"), distributionHandler.ListRules)
 			adminRoutes.POST("/distribution/rules", requirePerm("distribution:rule_manage"), distributionHandler.CreateRule)
 			adminRoutes.PUT("/distribution/rules/:id/status", requirePerm("distribution:rule_manage"), distributionHandler.UpdateRuleStatus)