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/GroupClass/EnrollmentRepositoryTest.php

@@ -56,6 +56,18 @@ class EnrollmentRepositoryTest extends TestCase
         self::assertSame(4, $this->repo->countActiveForOffering(7));
     }
 
+    public function testCountActiveForStudent(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/student_id = %d AND status = %s/'), 5, Enrollment::STATUS_ACTIVE)
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_var')->andReturn('2');
+
+        self::assertSame(2, $this->repo->countActiveForStudent(5));
+    }
+
     public function testHasActiveEnrollmentTrueWhenCountPositive(): void
     {
         $this->db->shouldReceive('prepare')