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/layout.tsx

@@ -3,12 +3,13 @@ import { headers } from "next/headers";
 import { auth } from "@/lib/auth/server";
 import { db, users, eq } from "@epicure/db";
 import Link from "next/link";
-import { Shield, Users, BookOpen, Settings, BarChart3, ClipboardList, HardDrive, Bot, ArrowLeft, Gauge } from "lucide-react";
+import { Shield, Users, BookOpen, Settings, BarChart3, ClipboardList, HardDrive, Bot, ArrowLeft, Gauge, Mail } from "lucide-react";
 import { cn } from "@/lib/utils";
 
 const adminNav = [
   { href: "/admin", label: "Overview", icon: BarChart3 },
   { href: "/admin/users", label: "Users", icon: Users },
+  { href: "/admin/invites", label: "Invites", icon: Mail },
   { href: "/admin/recipes", label: "Recipes", icon: BookOpen },
   { href: "/admin/tiers", label: "Tier Limits", icon: Gauge },
   { href: "/admin/audit-logs", label: "Audit Logs", icon: ClipboardList },