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/AdminMenu.php

@@ -8,6 +8,7 @@ use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\RegistrationController;
 use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Auth\StudentController;
 use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Booking\LessonController;
 use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
@@ -30,6 +31,7 @@ class AdminMenu {
 	private PolicyController $policyController;
 	private RegistrationController $registrationController;
 	private GroupClassController $groupClassController;
+	private StudentController $studentController;
 
 	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites, EnrollmentRepository $enrollments ) {
 		$this->availabilityController = new AvailabilityController( $availability, $offerings );
@@ -39,6 +41,7 @@ class AdminMenu {
 		$this->policyController       = new PolicyController( $policies, $policyVersions, $policyService );
 		$this->registrationController = new RegistrationController( $invites );
 		$this->groupClassController   = new GroupClassController( $enrollments, $offerings );
+		$this->studentController      = new StudentController( $bookings, $availability, $offerings, $enrollments );
 	}
 
 	public function register(): void {
@@ -122,6 +125,17 @@ class AdminMenu {
 			36
 		);
 
+		// Studio admin: browse students and their activity.
+		add_menu_page(
+			__( 'Students', 'unsupervised-schedular' ),
+			__( 'Students', 'unsupervised-schedular' ),
+			RoleManager::CAP_MANAGE_STUDENTS,
+			'us-students',
+			[ $this->studentController, 'renderPage' ],
+			'dashicons-id',
+			37
+		);
+
 		// Instructor: view their upcoming lessons.
 		add_menu_page(
 			__( 'My Lessons', 'unsupervised-schedular' ),