feat(admin): add image quota management UI on memind_adm 5174

Add image-quota admin pages and API wiring for plan defaults, ledger, and per-user
grants, plus local verify scripts and AGENTS.md note that this repo is the sole
admin UI surface.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/admin/AdminNav.tsx

@@ -21,7 +21,10 @@ const NAV_SECTIONS: NavSection[] = [
   },
   {
     label: '计费',
-    items: [{ to: '/billing', label: '计费中心', end: false }],
+    items: [
+      { to: '/billing', label: '计费中心', end: false },
+      { to: '/image-quota', label: '图片额度' },
+    ],
   },
   {
     label: '平台配置',