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]>

docs/features/offerings.md

@@ -20,6 +20,7 @@ An offering is anything a student can register for: a private-lesson type (30 or
 | `capacity`         | SMALLINT         | Group only — max enrolments; NULL for private               |
 | `term_start`       | DATE             | Group / term offerings — first day; NULL otherwise          |
 | `term_end`         | DATE             | Group / term offerings — last day; NULL otherwise           |
+| `class_time`       | TIME             | Group only — time of day each session starts; NULL otherwise |
 | `schedule_note`    | VARCHAR(191)     | Group only — human-readable schedule, e.g. "Tuesdays 4:00pm"|
 | `cancellation_cutoff_hours` | SMALLINT UNSIGNED | Optional per-offering cancellation cutoff in hours; NULL inherits the studio default (see `cancellation-cutoff.md`) |
 | `access_mode`      | VARCHAR(20)      | `public` (listed in the catalog) or `invite_only` (group classes hidden from the catalog — see `group-classes.md`) |
@@ -40,6 +41,28 @@ sessions** (`term_end = term_start + (N−1) weeks`, computed by
 student-facing class card shows the date (one-off) or the date range with the
 weekly session count.
 
+## Class Time and Sessions
+A group class also carries `class_time` — the time of day each session starts —
+validated by `Offering::normalizeTime()` (strict `H:i`/`H:i:s`; garbage leaves it
+NULL). `class_time` + `term_start`/`term_end` + `duration_minutes` together define
+the concrete session windows: `Offering::sessionWindows()` returns one
+`{start, end}` per session (weekly across the term, or a single window for a
+one-off), and returns an empty list unless date, time, and a positive duration are
+all set. These windows drive availability reconciliation (see **Instructor
+assignment** below and `group-classes.md`).
+
+## Instructor assignment
+Every offering has an owning `instructor_id`. A studio admin
+(`manage_instructors`) sees an **Instructor** picker on the offering form and may
+assign a group class to any instructor; a plain instructor never sees the picker
+and always owns the classes they create (the posted value is ignored for them, and
+updates never reassign the owner otherwise). When a group class is saved with an
+assigned instructor and a full schedule, `Offering\ClassSlotReconciler` clears that
+instructor's **open** availability slots overlapping each session so students can't
+book them, and reports any **already-booked** lesson that clashes as a conflict for
+the studio to resolve by hand (a booked lesson is never deleted). The result is
+surfaced as an admin notice after saving.
+
 ## Admin Interface
 Studio admin and instructors manage offerings under **Offerings** in wp-admin.
 - Studio admin (`manage_offerings`) manages offerings for any instructor.
@@ -61,11 +84,14 @@ Studio admin and instructors manage offerings under **Offerings** in wp-admin.
 
 ## Implementation
 - Repository: `Unsupervised\Schedular\Offering\OfferingRepository`
-- Model: `Unsupervised\Schedular\Offering\Offering`
+- Model: `Unsupervised\Schedular\Offering\Offering` (`normalizeTime`, `sessionWindows`)
 - Admin controller: `Unsupervised\Schedular\Offering\OfferingController`
-- REST endpoint: `Unsupervised\Schedular\Offering\OfferingEndpoint`
+- REST endpoint: `Unsupervised\Schedular\Offering\OfferingEndpoint` (public listing includes `instructor_name`)
+- Availability reconciliation: `Unsupervised\Schedular\Offering\ClassSlotReconciler` (uses `Availability\AvailabilityRepository::findOverlapping`)
 
 ## Tests
 - `tests/Unit/Offering/OfferingControllerTest.php`
 - `tests/Unit/Offering/OfferingRepositoryTest.php`
 - `tests/Unit/Offering/OfferingTest.php`
+- `tests/Unit/Offering/OfferingEndpointTest.php`
+- `tests/Unit/Offering/ClassSlotReconcilerTest.php`