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>

tests/Unit/Auth/StudentScheduleTest.php

@@ -0,0 +1,67 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Auth;
+
+use Unsupervised\Schedular\Auth\StudentSchedule;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class StudentScheduleTest extends TestCase
+{
+    public function testPartitionSplitsByNow(): void
+    {
+        $rows = [
+            ['start_dt' => '2026-06-10 09:00:00', 'label' => 'future-a'],
+            ['start_dt' => '2026-06-01 09:00:00', 'label' => 'past-a'],
+            ['start_dt' => '2026-06-20 09:00:00', 'label' => 'future-b'],
+            ['start_dt' => '2026-05-15 09:00:00', 'label' => 'past-b'],
+        ];
+
+        $result = StudentSchedule::partition($rows, '2026-06-08 12:00:00');
+
+        self::assertSame(['future-a', 'future-b'], array_column($result['upcoming'], 'label'));
+        self::assertSame(['past-a', 'past-b'], array_column($result['past'], 'label'));
+    }
+
+    public function testUpcomingSortedAscendingAndPastDescending(): void
+    {
+        $rows = [
+            ['start_dt' => '2026-06-20 09:00:00'],
+            ['start_dt' => '2026-06-10 09:00:00'],
+            ['start_dt' => '2026-05-01 09:00:00'],
+            ['start_dt' => '2026-05-30 09:00:00'],
+        ];
+
+        $result = StudentSchedule::partition($rows, '2026-06-08 00:00:00');
+
+        self::assertSame(
+            ['2026-06-10 09:00:00', '2026-06-20 09:00:00'],
+            array_column($result['upcoming'], 'start_dt')
+        );
+        self::assertSame(
+            ['2026-05-30 09:00:00', '2026-05-01 09:00:00'],
+            array_column($result['past'], 'start_dt')
+        );
+    }
+
+    public function testRowsWithoutStartDateFallIntoPast(): void
+    {
+        $rows = [
+            ['start_dt' => '', 'label' => 'no-slot'],
+            ['start_dt' => '2026-06-10 09:00:00', 'label' => 'future'],
+        ];
+
+        $result = StudentSchedule::partition($rows, '2026-06-08 00:00:00');
+
+        self::assertSame(['future'], array_column($result['upcoming'], 'label'));
+        self::assertSame(['no-slot'], array_column($result['past'], 'label'));
+    }
+
+    public function testEmptyInput(): void
+    {
+        $result = StudentSchedule::partition([], '2026-06-08 00:00:00');
+
+        self::assertSame([], $result['upcoming']);
+        self::assertSame([], $result['past']);
+    }
+}