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/RegistrationController.php

@@ -67,6 +67,26 @@ class RegistrationController {
 			}
 		}
 
+		if ( 'group_invite' === $action ) {
+			$expiresAt = $this->normalizeExpiry( sanitize_text_field( Val::string( wp_unslash( $_POST['expires_at'] ?? '' ) ) ) );
+
+			if ( null !== $expiresAt ) {
+				$rawToken = wp_generate_password( 32, false );
+
+				$this->invites->insert(
+					new Invite(
+						email:     '',
+						token:     Invite::hashToken( $rawToken ),
+						invitedBy: get_current_user_id(),
+						kind:      Invite::KIND_GROUP,
+						expiresAt: $expiresAt,
+					)
+				);
+
+				return $this->registrationLink( $rawToken );
+			}
+		}
+
 		if ( 'revoke' === $action ) {
 			$inviteId = absint( Val::int( $_POST['invite_id'] ?? 0 ) );
 			if ( $inviteId > 0 ) {
@@ -78,6 +98,23 @@ class RegistrationController {
 		return '';
 	}
 
+	/**
+	 * Validate a submitted group-link expiry date (strict `Y-m-d`, today or
+	 * later) and expand it to the end of that day; null when invalid or past.
+	 */
+	private function normalizeExpiry( string $date ): ?string {
+		$day = \DateTimeImmutable::createFromFormat( '!Y-m-d', $date );
+		if ( false === $day || $day->format( 'Y-m-d' ) !== $date ) {
+			return null;
+		}
+
+		if ( $date < Val::string( current_time( 'Y-m-d' ) ) ) {
+			return null;
+		}
+
+		return $date . ' 23:59:59';
+	}
+
 	/**
 	 * Build the registration URL for a raw invite token.
 	 */