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/Offering.php

@@ -53,6 +53,7 @@ class Offering {
 		public readonly ?int $capacity = null,
 		public readonly ?string $termStart = null,
 		public readonly ?string $termEnd = null,
+		public readonly ?string $classTime = null,
 		public readonly ?string $scheduleNote = null,
 		public readonly ?string $etransferEmail = null,
 		public readonly ?int $cancellationCutoffHours = null,
@@ -90,6 +91,69 @@ class Offering {
 		return ( new \DateTimeImmutable( $termStart ) )->modify( '+' . ( 7 * $weeks ) . ' days' )->format( 'Y-m-d' );
 	}
 
+	/**
+	 * Normalise a submitted time-of-day to canonical `H:i:s`, or null when it is
+	 * not a real time. Accepts the HTML `time` form (`H:i`, optionally with
+	 * seconds); anything else is rejected so garbage never reaches the TIME column.
+	 */
+	public static function normalizeTime( string $value ): ?string {
+		foreach ( [ 'H:i:s', 'H:i' ] as $format ) {
+			$time = \DateTimeImmutable::createFromFormat( '!' . $format, $value );
+			if ( false !== $time && $time->format( $format ) === $value ) {
+				return $time->format( 'H:i:s' );
+			}
+		}
+
+		return null;
+	}
+
+	/**
+	 * The concrete start/end datetimes of every session of this group class,
+	 * derived from the class date(s), the class time, and the duration. A weekly
+	 * class yields one window per week from `term_start` through `term_end`; a
+	 * one-off class yields a single window. Returns an empty list unless the
+	 * schedule is fully specified (date, time, and a positive duration), so it can
+	 * never fabricate a session window from partial data.
+	 *
+	 * @return list<array{start: string, end: string}>
+	 */
+	public function sessionWindows(): array {
+		if (
+			null === $this->termStart
+			|| null === $this->classTime
+			|| null === $this->durationMinutes
+			|| $this->durationMinutes <= 0
+		) {
+			return [];
+		}
+
+		$first = \DateTimeImmutable::createFromFormat( '!Y-m-d H:i:s', $this->termStart . ' ' . $this->classTime );
+		if ( false === $first ) {
+			return [];
+		}
+
+		$lastDay = null !== $this->termEnd ? $this->termEnd : $this->termStart;
+		$step    = new \DateInterval( 'PT' . $this->durationMinutes . 'M' );
+
+		$windows   = [];
+		$cursor    = $first;
+		$cursorDay = $cursor->format( 'Y-m-d' );
+
+		// Cap the walk at ten years of weeks so a term_end before term_start (or a
+		// bad value) can never spin into an unbounded loop.
+		for ( $i = 0; $i < 520 && $cursorDay <= $lastDay; $i++ ) {
+			$windows[] = [
+				'start' => $cursor->format( 'Y-m-d H:i:s' ),
+				'end'   => $cursor->add( $step )->format( 'Y-m-d H:i:s' ),
+			];
+
+			$cursor    = $cursor->modify( '+7 days' );
+			$cursorDay = $cursor->format( 'Y-m-d' );
+		}
+
+		return $windows;
+	}
+
 	public static function fromRow( \stdClass $row ): self {
 		return new self(
 			instructorId:    Val::int( $row->instructor_id ),
@@ -104,6 +168,7 @@ class Offering {
 			capacity:        Val::intOrNull( $row->capacity ),
 			termStart:       Val::stringOrNull( $row->term_start ),
 			termEnd:         Val::stringOrNull( $row->term_end ),
+			classTime:       Val::stringOrNull( $row->class_time ?? null ),
 			scheduleNote:    Val::stringOrNull( $row->schedule_note ),
 			etransferEmail:  Val::stringOrNull( $row->etransfer_email ),
 			cancellationCutoffHours: Val::intOrNull( $row->cancellation_cutoff_hours ),
@@ -137,6 +202,7 @@ class Offering {
 			'capacity'                  => $this->capacity,
 			'term_start'                => $this->termStart,
 			'term_end'                  => $this->termEnd,
+			'class_time'                => $this->classTime,
 			'schedule_note'             => $this->scheduleNote,
 			'cancellation_cutoff_hours' => $this->cancellationCutoffHours,
 			'access_mode'               => $this->accessMode,