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/invites/page.tsx

@@ -0,0 +1,34 @@
+import type { Metadata } from "next";
+import { listInvites } from "@/lib/invites";
+import { InvitesManager } from "@/components/admin/invites-manager";
+
+export const metadata: Metadata = { title: "Invites – Admin" };
+
+export default async function AdminInvitesPage() {
+  const invites = await listInvites();
+  const appUrl = process.env["BETTER_AUTH_URL"] ?? "http://localhost:3000";
+
+  return (
+    <div className="space-y-6">
+      <div>
+        <h1 className="text-2xl font-bold tracking-tight">Invites</h1>
+        <p className="text-muted-foreground text-sm mt-1">
+          Generate shareable signup links. Required when signups are disabled in{" "}
+          <span className="font-medium">Settings</span>.
+        </p>
+      </div>
+      <InvitesManager
+        invites={invites.map((i) => ({
+          id: i.id,
+          token: i.token,
+          email: i.email,
+          role: i.role,
+          tier: i.tier,
+          createdAt: i.createdAt.toISOString(),
+          expiresAt: i.expiresAt?.toISOString() ?? null,
+        }))}
+        appUrl={appUrl}
+      />
+    </div>
+  );
+}