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 <[email protected]>

src/GroupClass/EnrollmentRepository.php

@@ -49,6 +49,19 @@ class EnrollmentRepository {
 		);
 	}
 
+	/**
+	 * Count a student's active group-class enrolments.
+	 */
+	public function countActiveForStudent( int $studentId ): int {
+		return (int) $this->db->get_var(
+			$this->db->prepare(
+				"SELECT COUNT(*) FROM {$this->table} WHERE student_id = %d AND status = %s",
+				$studentId,
+				Enrollment::STATUS_ACTIVE
+			)
+		);
+	}
+
 	/**
 	 * Whether a student already holds an active enrolment in an offering.
 	 */