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>

docs/features/student-administration.md

@@ -3,7 +3,7 @@
 ## Overview
 A read-only studio-admin area to browse students and drill into one student's
 history and upcoming activity — lessons and group-class enrolments — without
-digging through individual records. (Status: planned — issue #22.)
+digging through individual records.
 
 ## Data Model
 No new tables. The views are composed from existing data:
@@ -34,10 +34,11 @@ Read-only in this iteration; cancel/edit actions are a possible follow-up.
 ## Implementation
 - Admin controller: `Unsupervised\Schedular\Auth\StudentController` (list + detail)
 - Templates: `templates/admin/students.php`, `templates/admin/student-detail.php`
-- Reuses `Booking\BookingRepository::findByStudent`,
+- Reuses `Booking\BookingRepository::findByStudent` + `countUpcomingForStudent`,
   `Availability\AvailabilityRepository::findById`,
   `Offering\OfferingRepository::findById`,
-  `GroupClass\EnrollmentRepository::findByStudent`
+  `GroupClass\EnrollmentRepository::findByStudent` + `countActiveForStudent`
+- Upcoming/past split: `Auth\StudentSchedule::partition()` (pure, unit-tested)
 - The upcoming/past split is extracted into a small pure helper so it is
   unit-testable (the controller itself follows the repo convention of not being
   unit-tested).