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/service/payment_order_lifecycle_test.go

@@ -208,6 +208,7 @@ func TestVerifyOrderByOutTradeNoBackfillsTradeNoFromPaidQuery(t *testing.T) {
 		nil,
 		client,
 		nil,
+		nil,
 	)
 	registry := payment.NewRegistry()
 	provider := &paymentOrderLifecycleQueryProvider{
@@ -308,6 +309,7 @@ func TestVerifyOrderByOutTradeNoRetriesZeroAmountPaidQueryOnce(t *testing.T) {
 		nil,
 		client,
 		nil,
+		nil,
 	)
 	registry := payment.NewRegistry()
 	provider := &paymentOrderLifecycleQueryProvider{
@@ -398,6 +400,7 @@ func TestVerifyOrderByOutTradeNoRejectsPaidQueryWithZeroAmount(t *testing.T) {
 		nil,
 		client,
 		nil,
+		nil,
 	)
 	registry := payment.NewRegistry()
 	provider := &paymentOrderLifecycleQueryProvider{
@@ -496,6 +499,7 @@ func TestVerifyOrderByOutTradeNoUsesOutTradeNoWhenPaymentTradeNoAlreadyExistsFor
 		nil,
 		client,
 		nil,
+		nil,
 	)
 	registry := payment.NewRegistry()
 	provider := &paymentOrderLifecycleQueryProvider{