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/api/v1/invites/[token]/route.ts

@@ -0,0 +1,15 @@
+import { NextRequest, NextResponse } from "next/server";
+import { findValidInvite } from "@/lib/invites";
+
+interface RouteContext {
+  params: Promise<{ token: string }>;
+}
+
+export async function GET(_req: NextRequest, { params }: RouteContext) {
+  const { token } = await params;
+  const invite = await findValidInvite(token);
+  if (!invite) {
+    return NextResponse.json({ valid: false });
+  }
+  return NextResponse.json({ valid: true, email: invite.email });
+}