feat(admin): Discounts and Website Settings sections

- /admin/discounts: list + create discount codes (kind, value, max uses, expiry)
  via /v1/discounts (backend has no edit/delete API yet)
- /admin/settings: key/value site settings with upsert + secret flag. The value
  column is jsonb, so values are JSON-encoded on save / decoded for display
- nav links + fa/en labels

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/[locale]/admin/layout.tsx

@@ -26,6 +26,8 @@ export default async function AdminLayout({
     { href: "/admin/ai", label: t("aiContent") },
     { href: "/admin/users", label: t("users") },
     { href: "/admin/plans", label: t("plans") },
+    { href: "/admin/discounts", label: t("discounts") },
+    { href: "/admin/settings", label: t("siteSettings") },
     { href: "/admin/nodes", label: t("nodes") },
     { href: "/admin/renders", label: t("renderQueue") },
   ];