invites: an admin lets one person register while registration stays closed

Until now adding a second person meant re-opening registration to the
whole internet while they signed up (#2939 §1). An admin now makes an
invite in Settings: a link that works once, expires (7 days by default,
1 to 30), and can be pinned to one email address. Only the token's hash
is stored, so the link is shown once.

POST /api/auth/register takes `invite`. Redemption is one conditional
UPDATE inside the transaction that creates the account, so two people
racing one link can't both get in, and a taken email leaves the invite
unused. Every refusal says "invalid or expired invite". The register
page reads ?invite= and opens even while registration is closed.

Refs #5172

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -112,7 +112,9 @@ router.beforeEach(async (to) => {
   if (to.meta.requiresServer && isDesktop()) {
     return { name: "board" };
   }
-  if (to.name === "register" && !config.allowRegistration) {
+  // An invite link opens the form while registration is closed; the server decides
+  // whether the invite holds.
+  if (to.name === "register" && !config.allowRegistration && !to.query.invite) {
     return { name: "login" };
   }
   if (to.meta.guestOnly && session.user) {