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]>

tests/Unit/Offering/OfferingEndpointTest.php

@@ -22,6 +22,7 @@ class OfferingEndpointTest extends TestCase
         parent::setUp();
 
         Functions\when('get_current_user_id')->justReturn(5);
+        Functions\when('get_userdata')->justReturn((object) ['display_name' => 'Ada Lovelace']);
 
         $this->repository = Mockery::mock(OfferingRepository::class);
         $this->access     = Mockery::mock(GroupAccessRepository::class);
@@ -88,6 +89,20 @@ class OfferingEndpointTest extends TestCase
         self::assertSame([], $data);
     }
 
+    public function testIndexIncludesInstructorNameForEachOffering(): void
+    {
+        $instructor = Mockery::mock(\WP_User::class);
+        $instructor->display_name = 'Ada Lovelace';
+        Functions\when('get_userdata')->justReturn($instructor);
+
+        $this->repository->shouldReceive('findAll')->andReturn([$this->group(1, Offering::ACCESS_PUBLIC)]);
+        $this->access->shouldReceive('findGrantedOfferingIds')->with(5)->andReturn([]);
+
+        $data = $this->endpoint->index(new \WP_REST_Request())->get_data();
+
+        self::assertSame('Ada Lovelace', $data[0]['instructor_name']);
+    }
+
     public function testIndexOmitsEtransferEmailFromPublicListing(): void
     {
         $this->repository->shouldReceive('findAll')->andReturn([