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>

templates/admin/students.php

@@ -0,0 +1,43 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+/**
+ * @var list<array{id: int, name: string, email: string, registered: string, upcoming: int, enrolments: int}> $students
+ * @var string $pageSlug
+ */
+?>
+<div class="wrap">
+    <h1><?php esc_html_e('Students', 'unsupervised-schedular'); ?></h1>
+
+    <?php if (empty($students)) : ?>
+        <p><?php esc_html_e('No students yet.', 'unsupervised-schedular'); ?></p>
+    <?php else : ?>
+        <table class="wp-list-table widefat fixed striped">
+            <thead>
+                <tr>
+                    <th><?php esc_html_e('Name', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Email', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Registered', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Upcoming lessons', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Active enrolments', 'unsupervised-schedular'); ?></th>
+                </tr>
+            </thead>
+            <tbody>
+                <?php foreach ($students as $student) : ?>
+                    <?php $detailUrl = add_query_arg(['page' => $pageSlug, 'student_id' => $student['id']], admin_url('admin.php')); ?>
+                    <tr>
+                        <td><a href="<?php echo esc_url($detailUrl); ?>"><?php echo esc_html($student['name']); ?></a></td>
+                        <td><?php echo esc_html($student['email']); ?></td>
+                        <td><?php echo esc_html($student['registered']); ?></td>
+                        <td><?php echo esc_html((string) $student['upcoming']); ?></td>
+                        <td><?php echo esc_html((string) $student['enrolments']); ?></td>
+                    </tr>
+                <?php endforeach; ?>
+            </tbody>
+        </table>
+    <?php endif; ?>
+</div>