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/OfferingTest.php

@@ -52,6 +52,82 @@ class OfferingTest extends TestCase
         self::assertSame('2026-09-08', Offering::weeklyTermEnd('2026-09-08', 0));
     }
 
+    public function testNormalizeTimeAcceptsHtmlTimeInput(): void
+    {
+        self::assertSame('16:30:00', Offering::normalizeTime('16:30'));
+        self::assertSame('09:00:00', Offering::normalizeTime('09:00:00'));
+    }
+
+    public function testNormalizeTimeRejectsGarbage(): void
+    {
+        self::assertNull(Offering::normalizeTime(''));
+        self::assertNull(Offering::normalizeTime('25:00'));
+        self::assertNull(Offering::normalizeTime('not-a-time'));
+    }
+
+    public function testSessionWindowsForOneOffClass(): void
+    {
+        $offering = new Offering(
+            instructorId: 3,
+            kind: Offering::KIND_GROUP_CLASS,
+            title: 'Recital',
+            durationMinutes: 90,
+            termStart: '2026-09-08',
+            termEnd: '2026-09-08',
+            classTime: '16:00:00',
+        );
+
+        self::assertSame(
+            [['start' => '2026-09-08 16:00:00', 'end' => '2026-09-08 17:30:00']],
+            $offering->sessionWindows()
+        );
+    }
+
+    public function testSessionWindowsWalksWeeklyTerm(): void
+    {
+        $offering = new Offering(
+            instructorId: 3,
+            kind: Offering::KIND_GROUP_CLASS,
+            title: 'Choir',
+            durationMinutes: 60,
+            termStart: '2026-09-08',
+            termEnd: '2026-09-22',
+            classTime: '16:00:00',
+        );
+
+        $windows = $offering->sessionWindows();
+
+        self::assertCount(3, $windows);
+        self::assertSame('2026-09-08 16:00:00', $windows[0]['start']);
+        self::assertSame('2026-09-22 16:00:00', $windows[2]['start']);
+        self::assertSame('2026-09-22 17:00:00', $windows[2]['end']);
+    }
+
+    public function testSessionWindowsEmptyWhenScheduleIncomplete(): void
+    {
+        // No class time.
+        $noTime = new Offering(
+            instructorId: 3,
+            kind: Offering::KIND_GROUP_CLASS,
+            title: 'Choir',
+            durationMinutes: 60,
+            termStart: '2026-09-08',
+            termEnd: '2026-09-08',
+        );
+        self::assertSame([], $noTime->sessionWindows());
+
+        // No duration.
+        $noDuration = new Offering(
+            instructorId: 3,
+            kind: Offering::KIND_GROUP_CLASS,
+            title: 'Choir',
+            termStart: '2026-09-08',
+            termEnd: '2026-09-08',
+            classTime: '16:00:00',
+        );
+        self::assertSame([], $noDuration->sessionWindows());
+    }
+
     public function testDefaults(): void
     {
         $offering = new Offering(1, Offering::KIND_GROUP_CLASS, 'Choir');