Add student administration view (studio-admin)

Implements #22: a read-only Students area for studio admins.

- StudentController (manage_students): a list of us_student users with
  upcoming-lesson and active-enrolment counts, each linking to a detail page
  showing account info, upcoming/past lessons (offering, instructor, status),
  and group-class enrolments.
- StudentSchedule::partition() — pure, unit-tested upcoming/past split.
- Repo counts: BookingRepository::countUpcomingForStudent and
  EnrollmentRepository::countActiveForStudent (single-query, tested).
- Templates: templates/admin/students.php, student-detail.php.
- Students admin menu wired in AdminMenu (no Plugin change — the repos were
  already available there).
- Docs: README status flipped to implemented; feature spec updated.

Payment history slots into the detail when Payments (#7) lands.

Tests: StudentScheduleTest + the two repo count tests. composer test (127),
cs, and PHPStan level 6 all pass.

Refs #22

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Auth/StudentController.php

@@ -0,0 +1,103 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+use Unsupervised\Schedular\Availability\AvailabilityRepository;
+use Unsupervised\Schedular\Booking\BookingRepository;
+use Unsupervised\Schedular\Booking\Lesson;
+use Unsupervised\Schedular\GroupClass\Enrollment;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
+use Unsupervised\Schedular\Offering\OfferingRepository;
+
+class StudentController {
+
+	public function __construct(
+		private BookingRepository $bookings,
+		private AvailabilityRepository $availability,
+		private OfferingRepository $offerings,
+		private EnrollmentRepository $enrollments,
+	) {}
+
+	public function renderPage(): void {
+		if ( ! current_user_can( RoleManager::CAP_MANAGE_STUDENTS ) ) {
+			wp_die( esc_html__( 'You do not have permission to view students.', 'unsupervised-schedular' ) );
+		}
+
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- read-only student selector.
+		$studentId = absint( $_GET['student_id'] ?? 0 );
+		$student   = $studentId > 0 ? get_userdata( $studentId ) : false;
+
+		if ( $student && in_array( RoleManager::STUDENT, (array) $student->roles, true ) ) {
+			$this->renderDetail( $student );
+			return;
+		}
+
+		$students = array_map(
+			fn( \WP_User $user ): array => [
+				'id'         => (int) $user->ID,
+				'name'       => $user->display_name,
+				'email'      => $user->user_email,
+				'registered' => $user->user_registered,
+				'upcoming'   => $this->bookings->countUpcomingForStudent( (int) $user->ID ),
+				'enrolments' => $this->enrollments->countActiveForStudent( (int) $user->ID ),
+			],
+			get_users(
+				[
+					'role'    => RoleManager::STUDENT,
+					'orderby' => 'display_name',
+					'order'   => 'ASC',
+				]
+			)
+		);
+
+		$pageSlug = 'us-students';
+		include USC_PLUGIN_DIR . 'templates/admin/students.php';
+	}
+
+	private function renderDetail( \WP_User $student ): void {
+		$now  = current_time( 'mysql' );
+		$rows = array_map(
+			fn( Lesson $lesson ): array => $this->lessonRow( $lesson ),
+			$this->bookings->findByStudent( (int) $student->ID )
+		);
+
+		$schedule = StudentSchedule::partition( $rows, $now );
+		$upcoming = $schedule['upcoming'];
+		$past     = $schedule['past'];
+
+		$enrolments = array_map(
+			function ( Enrollment $enrollment ): array {
+				$offering = $this->offerings->findById( $enrollment->offeringId );
+
+				return [
+					'offering' => $offering ? $offering->title : (string) $enrollment->offeringId,
+					'status'   => $enrollment->status,
+				];
+			},
+			$this->enrollments->findByStudent( (int) $student->ID )
+		);
+
+		$backUrl = admin_url( 'admin.php?page=us-students' );
+		include USC_PLUGIN_DIR . 'templates/admin/student-detail.php';
+	}
+
+	/**
+	 * Build a display row for a lesson (slot time, offering, instructor, status).
+	 *
+	 * @return array<string, mixed>
+	 */
+	private function lessonRow( Lesson $lesson ): array {
+		$slot       = $this->availability->findById( $lesson->slotId );
+		$offering   = null !== $lesson->offeringId ? $this->offerings->findById( $lesson->offeringId ) : null;
+		$instructor = get_userdata( $lesson->instructorId );
+
+		return [
+			'start_dt'   => $slot ? $slot->startDt : '',
+			'end_dt'     => $slot ? $slot->endDt : '',
+			'offering'   => $offering ? $offering->title : '—',
+			'instructor' => $instructor ? $instructor->display_name : (string) $lesson->instructorId,
+			'status'     => $lesson->status,
+		];
+	}
+}