feat(provisioning): game-account signup, admin email invites, unlink (2.0)

Phase 5: the account-provisioning backend — link-only stays, plus hybrid
self-signup, an admin email-invite tool, and site-side unlink.

- uoLinkClient.createAccount / unlinkAccount (v2). Password is forwarded to the
  shard (hashed there) and never stored/logged; the end-user browser IP is passed
  for the shard's per-IP cap; actor is stamped server-side.
- Hybrid signup: POST /player/shard/account provisions a game account (its own
  username + password) for the signed-in user and mirrors the link locally. Gated
  by the new game_account_signup setting AND the shard's own mode (mapped 403/409/
  429/400/503). Serves both self-serve signup and the invite-accept game step.
- Email invites: user_invites table (sha256 token hash, single-use, expiring);
  invites model + admin CRUD (POST/GET/DELETE /admin/invites, admin-only) +
  mailer.sendInvite (falls back to returning the accept link if email is off);
  public token-gated accept (GET /auth/invite/:token, POST .../accept) creates the
  user at the invite's preset role and logs them in, bypassing the registration
  gate. Accept is race-safe (atomic single-use; rolls back the user if it loses).
- Admin unlink: DELETE /admin/users/:id/shard/link/:account (admin-only) + local
  mirror drop; account.unlinked ingest reconciles the mirror when a player runs
  [unlink in game. account.audit / account.unlinked are logged (admin channel
  only — never on the public SSE allowlist).

Tests: invites model (hashing, single-use, expiry, revoke) + account.* ingest
reconcile/visibility. Full suite 193/193; swagger regenerated.

Refs .plans/protocol2-integration.md (Phase 5).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/model/settings/settings.model.js

@@ -32,6 +32,13 @@ function registrationFlags(mode) {
   }
 }
 
+// Game-account signup (Protocol 2.0 hybrid mode). Off unless an admin opts in;
+// the shard's own signup mode still has the final say when we call the sidecar.
+const GAME_SIGNUP_KEY = 'game_account_signup'
+async function isGameAccountSignupEnabled() {
+  return (await settingsDb.get(GAME_SIGNUP_KEY)) === 'enabled'
+}
+
 async function get(key) {
   return settingsDb.get(key)
 }
@@ -78,4 +85,6 @@ module.exports = {
   REGISTRATION_MODES,
   getRegistrationMode,
   registrationFlags,
+  GAME_SIGNUP_KEY,
+  isGameAccountSignupEnabled,
 }