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/app/admin/layout.tsx

@@ -18,6 +18,7 @@ export default async function AdminLayout({ children }: { children: ReactNode })
           <Link href="/admin/users">Users</Link>
           <Link href="/admin/articles">Articles</Link>
           <Link href="/admin/bans">Bans</Link>
+          <Link href="/admin/settings">Settings</Link>
         </nav>
       </aside>
       <section>{children}</section>