feat: add redeem code affiliate rebate, batch concurrency API, and markdown page rendering

1. Redeem code affiliate rebate: balance-type redeem codes now trigger
   invite rebate for the inviter. Payment fulfillment uses context key
   to prevent double-rebate.

2. Batch concurrency update: new POST /admin/users/batch-concurrency
   endpoint supporting mode=set/add with all=true for all users.

3. Markdown page rendering: new GET /api/v1/pages/:slug API serves local
   .md files. Custom menu items with url="md:slug" render markdown with
   collapsible TOC sidebar, scroll spy, and copy buttons on code blocks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/server/routes/admin.go

@@ -228,6 +228,7 @@ func registerUserManagementRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		users.GET("/:id/balance-history", h.Admin.User.GetBalanceHistory)
 		users.POST("/:id/replace-group", h.Admin.User.ReplaceGroup)
 		users.GET("/:id/rpm-status", h.Admin.User.GetUserRPMStatus)
+		users.POST("/batch-concurrency", h.Admin.User.BatchUpdateConcurrency)
 
 		// User attribute values
 		users.GET("/:id/attributes", h.Admin.UserAttribute.GetUserAttributes)