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/router.go

@@ -112,4 +112,6 @@ func registerRoutes(
 	routes.RegisterAdminRoutes(v1, h, adminAuth)
 	routes.RegisterGatewayRoutes(r, h, apiKeyAuth, apiKeyService, subscriptionService, opsService, settingService, cfg)
 	routes.RegisterPaymentRoutes(v1, h.Payment, h.PaymentWebhook, h.Admin.Payment, jwtAuth, adminAuth, settingService)
+
+	handler.RegisterPageRoutes(v1, cfg.Pricing.DataDir)
 }