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>

packages/db/src/migrations/meta/_journal.json

@@ -106,6 +106,13 @@
       "when": 1782984288632,
       "tag": "0014_late_marvex",
       "breakpoints": true
+    },
+    {
+      "idx": 15,
+      "version": "7",
+      "when": 1783105214683,
+      "tag": "0015_aromatic_lester",
+      "breakpoints": true
     }
   ]
 }