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/Availability/AvailabilityRepository.php

@@ -181,6 +181,28 @@ class AvailabilityRepository {
 		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
 	}
 
+	/**
+	 * An instructor's slots (booked and unbooked) that overlap a time window —
+	 * they share any time with the half-open interval [$start, $end). Used when a
+	 * group class is scheduled to find the private-booking slots that collide with
+	 * it, so open ones can be cleared and booked ones flagged as conflicts.
+	 *
+	 * @return list<AvailabilitySlot>
+	 */
+	public function findOverlapping( int $instructorId, string $start, string $end ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				'SELECT * FROM %i WHERE instructor_id = %d AND start_dt < %s AND end_dt > %s ORDER BY start_dt ASC',
+				$this->table,
+				$instructorId,
+				$end,
+				$start
+			)
+		);
+
+		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
+	}
+
 	public function findById( int $id ): ?AvailabilitySlot {
 		$row = $this->db->get_row(
 			$this->db->prepare( 'SELECT * FROM %i WHERE id = %d', $this->table, $id )