Add multi-use group invite links with expiry and auto-approval on email confirmation

A studio admin can generate a shareable group invite link (e.g. for a
newsletter) from the Invites page, choosing a required expiry date. Anyone
with the link may register while it is valid, in any registration mode: the
form collects their own email, they must confirm it via the usual hashed
token, and confirming approves the account immediately — group signups never
enter the Pending Students queue.

- us_invites grows kind (personal/group) and expires_at; an explicit expiry
  wins over the personal 14-day window. Group links stay pending (multi-use)
  until revoked or expired.
- RegistrationPage: group signups create the account pending with the
  us_auto_approve marker and send the confirmation email; no auto-login.
- EmailConfirmationHandler: auto-approve accounts are approved on
  confirmation, emailed the approved notice, and redirected to a new
  us_confirmed=ready notice with a sign-in link.

Closes #77

Co-Authored-By: Claude Fable 5 <[email protected]>

src/Auth/EmailConfirmationHandler.php

@@ -55,6 +55,21 @@ class EmailConfirmationHandler {
 		RegistrationStatus::confirmEmail( $userId );
 
 		$user = get_user_by( 'id', $userId );
+
+		// Group invite link signups skip the admin review queue: confirming the
+		// email approves the account on the spot, so the student can sign in
+		// immediately instead of waiting for a studio admin.
+		if ( RegistrationStatus::isAutoApprove( $userId ) ) {
+			RegistrationStatus::approve( $userId );
+
+			if ( $user instanceof \WP_User ) {
+				$this->mailer->sendApproved( $user );
+			}
+
+			wp_safe_redirect( add_query_arg( 'us_confirmed', 'ready', $base ) );
+			exit;
+		}
+
 		if ( $user instanceof \WP_User ) {
 			$this->mailer->notifyAdminsPending( $user );
 		}