Collect a birth year instead of a full date of birth

Signup and the profile page now ask for a four-digit year between 1900 and
the current year. Anything else — a short year, a full date, a year in the
future — is discarded rather than stored, so a typo cannot leave a nonsense
age on the record.

The year lives in a new us_birth_year user meta rather than reusing
us_date_of_birth, which would have left one key holding two formats. The old
key is not migrated in bulk. Instead GuardianService handles it in two
halves: birthYear() falls back to the year of the old date when the new key
is absent, so a student added before this change still shows one, and
setBirthYear() deletes the old date on every save.

That deletion is what makes the fallback safe rather than merely tidy.
Without it, clearing the birth year on a student who predates the change
would leave the old date behind for the fallback to read straight back, and
the year could never be cleared at all.

Stored in user meta, so no Schema.php change and no USC_VERSION bump.

Closes #147

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

docs/features/parent-guardian-accounts.md

@@ -67,11 +67,28 @@ requires migrating every existing row.
 never be linked twice.
 
 The table is a link table, not a child record: the child's **name** is their
-`display_name` on `wp_users`, and their date of birth is the `us_date_of_birth`
+`display_name` on `wp_users`, and their birth year is the `us_birth_year`
 user meta. Keeping them on the user row means the admin student screens,
 `get_users()` ordering, and every existing `student_id` lookup keep working with
 no special-casing.
 
+### The legacy `us_date_of_birth` meta
+
+This feature originally collected a full date of birth in `us_date_of_birth`.
+Nothing writes that key any more. It is handled entirely inside
+`GuardianService`:
+
+- **Read** — `birthYear()` falls back to the year of the old date when
+  `us_birth_year` is absent, so a child added before the change still shows one
+  without a migration step.
+- **Write** — `setBirthYear()` deletes `us_date_of_birth` on *every* save,
+  including a save that clears the year. Without that the fallback would
+  resurrect the old date on the next read and the year could never be cleared.
+
+The upshot is a lazy migration: a child's full date survives until their record
+is next edited, then goes for good. There is no bulk purge — a site that wants
+the remaining old dates gone should delete the `us_date_of_birth` meta directly.
+
 v1 is deliberately **one guardian per child**: `GuardianRepository::insert()`
 refuses to link a child that already has a guardian. The unique key and the
 guardian-side lookups already support many-to-many, so adding a second guardian
@@ -130,7 +147,9 @@ least one child name.
 
 Per child the form collects:
 - **Name** (required)
-- **Date of birth** (optional, `us_date_of_birth` meta)
+- **Birth year** (optional, `us_birth_year` meta) — a four-digit year between
+  1900 and the current year. Anything else is discarded rather than stored, so
+  a typo cannot leave a nonsense age on the record.
 - **Every account-scope registration question** (`Registration\Question`,
   `SCOPE_ACCOUNT`) — asked once per child, not once per guardian, because in
   practice they describe the student (instrument, level, school). The guardian
@@ -173,12 +192,12 @@ child.
 ## Managing children
 
 `[us_family]` (block: **Profile**) renders the guardian's manage-children screen:
-list the children, add one, edit a name/date of birth, remove one.
+list the children, add one, edit a name/birth year, remove one.
 
 - **Add** creates another accountless child user and links it. Account-scope
   questions are asked here too, so a child added later carries the same
   information as one added at signup.
-- **Edit** updates `display_name` and `us_date_of_birth`.
+- **Edit** updates `display_name` and `us_birth_year`.
 - **Remove** unlinks the child and **deletes the child user**, but only when the
   child has no lessons and no enrolments — a child with history is refused, so
   removing one can never orphan a lesson, payment or credit