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/Booking/BookingRepository.php

@@ -111,6 +111,26 @@ class BookingRepository {
 		return array_map( Lesson::fromRow( ... ), $rows ?? [] );
 	}
 
+	/**
+	 * Count a student's upcoming, non-cancelled lessons (slot in the future).
+	 */
+	public function countUpcomingForStudent( int $studentId ): int {
+		$avTable = str_replace( 'us_lessons', 'us_availability', $this->table );
+
+		return (int) $this->db->get_var(
+			$this->db->prepare(
+				"SELECT COUNT(*) FROM {$this->table} l
+                 JOIN {$avTable} a ON a.id = l.slot_id
+                 WHERE l.student_id = %d
+                   AND l.status != %s
+                   AND a.start_dt >= %s",
+				$studentId,
+				Lesson::STATUS_CANCELLED,
+				current_time( 'mysql' )
+			)
+		);
+	}
+
 	/**
 	 * All lessons for a student.
 	 *