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/middleware/admin_auth_test.go

@@ -198,6 +198,9 @@ func (s *stubUserRepo) UpdateConcurrency(ctx context.Context, id int64, amount i
 	panic("unexpected UpdateConcurrency call")
 }
 
+func (s *stubUserRepo) BatchSetConcurrency(context.Context, []int64, int) (int, error) { return 0, nil }
+func (s *stubUserRepo) BatchAddConcurrency(context.Context, []int64, int) (int, error) { return 0, nil }
+
 func (s *stubUserRepo) ExistsByEmail(ctx context.Context, email string) (bool, error) {
 	panic("unexpected ExistsByEmail call")
 }