Add admin CMS Settings editor (website_settings)

/admin/settings: list all website_settings, inline value edit, add/overwrite,
delete; staff-gated server actions that bust the siteSettings cache after each
write. Admin nav extended (Settings).

Verified: tsc exit 0, vitest 48/48, next build exit 0.

src/actions/admin-settings.ts

@@ -0,0 +1,40 @@
+"use server";
+
+import { revalidatePath } from "next/cache";
+import { requireStaff } from "@/lib/admin/guard";
+import { prisma } from "@/lib/prisma";
+import { siteSettings } from "@/lib/services/site-settings";
+
+export async function updateSetting(formData: FormData): Promise<void> {
+  await requireStaff();
+  const key = String(formData.get("key") ?? "").trim();
+  const value = String(formData.get("value") ?? "");
+  if (!key) return;
+  await prisma.websiteSetting.update({ where: { key }, data: { value } });
+  siteSettings.reload();
+  revalidatePath("/admin/settings");
+}
+
+export async function createSetting(formData: FormData): Promise<void> {
+  await requireStaff();
+  const key = String(formData.get("key") ?? "").trim().slice(0, 255);
+  const value = String(formData.get("value") ?? "");
+  const comment = String(formData.get("comment") ?? "").trim().slice(0, 255);
+  if (!key) return;
+  await prisma.websiteSetting.upsert({
+    where: { key },
+    update: { value },
+    create: { key, value, comment: comment || null },
+  });
+  siteSettings.reload();
+  revalidatePath("/admin/settings");
+}
+
+export async function deleteSetting(formData: FormData): Promise<void> {
+  await requireStaff();
+  const key = String(formData.get("key") ?? "").trim();
+  if (!key) return;
+  await prisma.websiteSetting.delete({ where: { key } });
+  siteSettings.reload();
+  revalidatePath("/admin/settings");
+}