Add group-class scheduling, instructor assignment, and details/invite management

Group classes now carry a specific class time (alongside date and duration)
and an assigned instructor:

- Schema: add `class_time` (TIME) to `us_offerings`; `Offering` gains
  `normalizeTime`/`sessionWindows`. (Rides the pending 1.0.0->1.1.0 dbDelta
  upgrade, so no version bump.)
- Offering form: class-time field, plus a studio-admin instructor picker
  (plain instructors always own their own classes).
- `ClassSlotReconciler`: assigning an instructor clears their open booking
  slots overlapping each session and flags already-booked lessons that clash
  (a booked lesson is never deleted). Uses new
  `AvailabilityRepository::findOverlapping`.
- Front end: `GET /offerings` exposes `instructor_name`; the enrolment page
  shows who teaches each class and when it meets.

Back-office group-class views redesigned:

- Instructor **My Group Classes** and studio-admin **Group Classes** are now
  per-class summaries with enrolment counts, not flat student lists.
- Each links through (`?class_id=<id>`) to a per-class **details page**
  (schedule panel, roster with payment status, and — for invite-only classes
  — the add/make-available/invite-by-email controls). Invite-only membership
  is managed entirely from this page.
- Invite actions are allowed for the class's owning instructor or any
  `view_all_lessons` studio admin, so an owner-operator (studio admin who also
  teaches) can reach every class's roster and invites from the Group Classes
  page.

Tests: composer test (508), composer lint, composer cs all pass.

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

src/Offering/OfferingEndpoint.php

@@ -77,8 +77,25 @@ class OfferingEndpoint {
 			$offerings[] = $granted;
 		}
 
-		// Public listing: omit the private e-transfer destination email.
-		return new \WP_REST_Response( array_map( fn( Offering $o ) => $o->toArray( includeEtransferEmail: false ), $offerings ), 200 );
+		// Public listing: omit the private e-transfer destination email, and
+		// attach the assigned instructor's display name so the front end can show
+		// students who teaches each class.
+		return new \WP_REST_Response( array_map( [ $this, 'present' ], $offerings ), 200 );
+	}
+
+	/**
+	 * A public-facing offering array with the assigned instructor's display name
+	 * added (empty when the instructor account no longer exists).
+	 *
+	 * @return array<string, mixed>
+	 */
+	private function present( Offering $offering ): array {
+		$out  = $offering->toArray( includeEtransferEmail: false );
+		$user = get_userdata( $offering->instructorId );
+
+		$out['instructor_name'] = $user instanceof \WP_User ? $user->display_name : '';
+
+		return $out;
 	}
 
 	/**