Add invite-only group classes

Group classes can now be marked invite-only (us_offerings.access_mode).
Invite-only classes are hidden from the public catalog and reachable only
when the instructor lets someone in via one of three paths, managed from
My Lessons -> My Group Classes:

- Add students directly: enrols them now with a pending payment.
- Make available: grants registered students access to self-enrol through
  the normal paid flow (multi-select, emailed a notice).
- Invite by email: tokenised registration invite tied to the class for a
  non-account address; after they register the class becomes enrollable.
  Reuses an existing pending invite instead of sending a second link.

New us_group_access table records grants; GET /offerings merges granted
invite-only classes for the caller; enrolment requires a grant
(403 invite_required) and flips it to enrolled on success.

composer test (487), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Auth/RegistrationMailer.php

@@ -105,6 +105,56 @@ class RegistrationMailer {
 		return (bool) wp_mail( $email, $subject, $body );
 	}
 
+	/**
+	 * Tell a registered student they have been given access to an invite-only
+	 * group class and can now enrol. Returns false when there is no recipient.
+	 */
+	public function sendClassAccessGranted( \WP_User $user, string $className ): bool {
+		if ( '' === (string) $user->user_email ) {
+			return false;
+		}
+
+		$subject = sprintf(
+			/* translators: %s: class title */
+			__( 'You have been invited to %s', 'unsupervised-schedular' ),
+			$className
+		);
+		$body = sprintf(
+			/* translators: 1: class title, 2: site name, 3: login URL */
+			__( "You have been given access to the group class \"%1\$s\" at %2\$s.\n\nLog in and open the group classes page to enrol:\n%3\$s", 'unsupervised-schedular' ),
+			$className,
+			$this->siteName(),
+			wp_login_url()
+		);
+
+		return (bool) wp_mail( $user->user_email, $subject, $body );
+	}
+
+	/**
+	 * Email a tokenised registration link to someone invited to a group class who
+	 * does not yet have an account. Returns false when there is no recipient.
+	 */
+	public function sendClassInvite( string $email, string $link, string $className ): bool {
+		if ( '' === $email ) {
+			return false;
+		}
+
+		$subject = sprintf(
+			/* translators: %s: class title */
+			__( 'You are invited to join %s', 'unsupervised-schedular' ),
+			$className
+		);
+		$body = sprintf(
+			/* translators: 1: class title, 2: site name, 3: registration URL */
+			__( "You have been invited to the group class \"%1\$s\" at %2\$s.\n\nCreate your account using this link, then choose to enrol in the class:\n%3\$s", 'unsupervised-schedular' ),
+			$className,
+			$this->siteName(),
+			$link
+		);
+
+		return (bool) wp_mail( $email, $subject, $body );
+	}
+
 	private function siteName(): string {
 		$name = (string) get_bloginfo( 'name' );