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/handler/admin/admin_service_stub_test.go

@@ -175,6 +175,10 @@ func (s *stubAdminService) UpdateUserBalance(ctx context.Context, userID int64,
 	return &user, nil
 }
 
+func (s *stubAdminService) BatchUpdateConcurrency(ctx context.Context, userIDs []int64, value int, mode string) (int, error) {
+	return len(userIDs), nil
+}
+
 func (s *stubAdminService) GetUserAPIKeys(ctx context.Context, userID int64, page, pageSize int, sortBy, sortOrder string) ([]service.APIKey, int64, error) {
 	return s.apiKeys, int64(len(s.apiKeys)), nil
 }