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/dto/settings.go

@@ -11,6 +11,7 @@ type CustomMenuItem struct {
 	Label      string `json:"label"`
 	IconSVG    string `json:"icon_svg"`
 	URL        string `json:"url"`
+	PageSlug   string `json:"page_slug,omitempty"`
 	Visibility string `json:"visibility"` // "user" or "admin"
 	SortOrder  int    `json:"sort_order"`
 }