feat: signup toggle, invite links, admin-created users

- New invites table: token-gated signup, optional email lock,
  role/tier override, single-use, expiry.
- SIGNUPS_DISABLED site setting toggle at /admin/settings.
- databaseHooks.user.create gate in auth/server.ts blocks new account
  creation (email + Google OAuth) when disabled unless a valid invite
  cookie is present; applies invite role/tier and marks it consumed.
- /admin/invites: create/list/revoke shareable invite links.
- /admin/users: "Create user" dialog — admin sets email/role/tier,
  account is pre-verified, user gets a set-password email (admin
  never sees a password).
- Signup page reads ?invite=, validates via public
  /api/v1/invites/[token], locks the form when signups are closed
  and no valid invite is present.
- proxy.ts: allowlist /api/v1/invites/ for anonymous invite checks.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/admin/settings/page.tsx

@@ -1,6 +1,7 @@
 import type { Metadata } from "next";
-import { getAllSiteSettings } from "@/lib/site-settings";
+import { getAllSiteSettings, isSignupsDisabled } from "@/lib/site-settings";
 import { AdminSettingsForm } from "@/components/admin/admin-settings-form";
+import { SignupsToggle } from "@/components/admin/signups-toggle";
 
 export const metadata: Metadata = { title: "Site Settings – Admin" };
 
@@ -24,6 +25,7 @@ const SETTING_GROUPS = [
 
 export default async function AdminSettingsPage() {
   const settings = await getAllSiteSettings();
+  const signupsDisabled = await isSignupsDisabled();
 
   return (
     <div className="space-y-8">
@@ -35,6 +37,8 @@ export default async function AdminSettingsPage() {
         </p>
       </div>
 
+      <SignupsToggle initialDisabled={signupsDisabled} />
+
       {SETTING_GROUPS.map((group) => (
         <AdminSettingsForm key={group.title} group={group} settings={settings} />
       ))}