Show "Booked by" in the Account section of a student's detail page

The parent/guardian was only named further down under Profile, where it
reads as background rather than as an account fact, and only when there
was one — so a page with no such line was ambiguous between "books for
themselves" and "the lookup found nothing".

It now sits in the Account table beside display name and email, as the
guardian's name linked to their own detail page, and always renders: a
student who books for themselves says so outright. No email address —
theirs is one click away on their own page, and repeating it here only
makes the row harder to scan. The Profile section keeps only the note
explaining the placeholder email, which is a different point.

Tests: composer test (863), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 5 <[email protected]>

CHANGELOG.md

@@ -16,6 +16,7 @@ each change under the current top section as you work.
 ### Added
 - **Group classes now appear in "Your upcoming lessons".** A class is stored as a term rather than as bookable slots, so nothing that listed lessons could ever show one — a student whose whole term was a group class saw an empty schedule, and an instructor teaching one saw nothing on their My Lessons page. Each remaining session of a class you are enrolled in now sorts in among your lessons by date, labelled **group class**; instructors see every session of the classes they teach, one row per session however many students are in it. A session has no Cancel button, because there is no such thing as cancelling one date of a term — withdrawing from the class is still done from the class page.
 - A class you are enrolled in shows up **whether or not its schedule is pinned to a clock**. Class time and duration are both optional on the offering form, and the schedule note is there so a studio can simply write "Tuesdays 4:00pm" — so a class with a time but no duration lists its dates and says when each session starts rather than guessing when it ends, and a class with no time at all gets a single row carrying its schedule note (or its term dates) where the time would go. Only a class whose last day has passed drops off the list.
+- A student's admin detail page now shows **Booked by** in the Account section — the name of the parent or guardian who books and pays for them, linked to their own page. It was only stated further down under Profile, and only when there was one; the row is now always there, saying in words when a student books for themselves.
 - The same group-class sessions now appear in **Upcoming lessons** on a student's admin detail page, so one table answers "what are they booked into next week?". Only upcoming ones — the **Group-class enrolments** table below already holds the history.
 - **A policy can be renamed.** The title was fixed at creation, so a typo or a change of wording meant creating a second policy and re-collecting everyone's acceptance. Renaming changes only what students read above the policy text: the slug stays put, so every version already accepted stays attached.