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/Offering/Offering.php

@@ -27,6 +27,19 @@ class Offering {
 	 */
 	public const VALID_BILLING_MODES = [ self::BILLING_ONE_TIME, self::BILLING_FULL_TERM ];
 
+	/** Listed in the public catalogue; anyone with `book_lesson` may enrol. */
+	public const ACCESS_PUBLIC = 'public';
+
+	/** Hidden from the catalogue; only invited/added students may enrol (group classes). */
+	public const ACCESS_INVITE_ONLY = 'invite_only';
+
+	/**
+	 * All valid access modes.
+	 *
+	 * @var list<string>
+	 */
+	public const VALID_ACCESS_MODES = [ self::ACCESS_PUBLIC, self::ACCESS_INVITE_ONLY ];
+
 	public function __construct(
 		public readonly int $instructorId,
 		public readonly string $kind,
@@ -43,10 +56,19 @@ class Offering {
 		public readonly ?string $scheduleNote = null,
 		public readonly ?string $etransferEmail = null,
 		public readonly ?int $cancellationCutoffHours = null,
+		public readonly string $accessMode = self::ACCESS_PUBLIC,
 		public readonly bool $isActive = true,
 		public readonly ?int $id = null,
 	) {}
 
+	/**
+	 * Whether the offering is hidden from the public catalogue and reachable
+	 * only by invited or directly-added students.
+	 */
+	public function isInviteOnly(): bool {
+		return self::ACCESS_INVITE_ONLY === $this->accessMode;
+	}
+
 	/**
 	 * Normalise a submitted term date to canonical `Y-m-d`, or null when it is
 	 * not a real calendar date. Round-trips through DateTimeImmutable so
@@ -85,6 +107,7 @@ class Offering {
 			scheduleNote:    Val::stringOrNull( $row->schedule_note ),
 			etransferEmail:  Val::stringOrNull( $row->etransfer_email ),
 			cancellationCutoffHours: Val::intOrNull( $row->cancellation_cutoff_hours ),
+			accessMode:      '' !== Val::string( $row->access_mode ?? '' ) ? Val::string( $row->access_mode ) : self::ACCESS_PUBLIC,
 			isActive:        Val::bool( $row->is_active ),
 			id:              Val::int( $row->id ),
 		);
@@ -116,6 +139,7 @@ class Offering {
 			'term_end'                  => $this->termEnd,
 			'schedule_note'             => $this->scheduleNote,
 			'cancellation_cutoff_hours' => $this->cancellationCutoffHours,
+			'access_mode'               => $this->accessMode,
 			'is_active'                 => $this->isActive,
 		];