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/Auth/StudentSchedule.php

@@ -0,0 +1,40 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+/**
+ * Pure helper for splitting a student's dated rows into upcoming and past.
+ */
+class StudentSchedule {
+
+	/**
+	 * Partition rows (each with a `start_dt` string in `Y-m-d H:i:s`) relative to
+	 * `$now`. Upcoming rows are sorted ascending, past rows descending. Rows
+	 * without a usable `start_dt` fall into `past`.
+	 *
+	 * @param list<array<string, mixed>> $rows
+	 * @return array{upcoming: list<array<string, mixed>>, past: list<array<string, mixed>>}
+	 */
+	public static function partition( array $rows, string $now ): array {
+		$upcoming = [];
+		$past     = [];
+
+		foreach ( $rows as $row ) {
+			$start = (string) ( $row['start_dt'] ?? '' );
+			if ( '' !== $start && $start >= $now ) {
+				$upcoming[] = $row;
+			} else {
+				$past[] = $row;
+			}
+		}
+
+		usort( $upcoming, static fn( array $a, array $b ): int => strcmp( (string) ( $a['start_dt'] ?? '' ), (string) ( $b['start_dt'] ?? '' ) ) );
+		usort( $past, static fn( array $a, array $b ): int => strcmp( (string) ( $b['start_dt'] ?? '' ), (string) ( $a['start_dt'] ?? '' ) ) );
+
+		return [
+			'upcoming' => array_values( $upcoming ),
+			'past'     => array_values( $past ),
+		];
+	}
+}