Let parents register once and book for their children

A parent registers once and manages lessons for one or more children, who
need no login of their own. A child is a real wp_users row with the student
role but no usable login — so student_id keeps meaning "a WordPress user"
on every table, and booking, credits, policies and enrolments work unchanged.
A us_guardians link table maps guardian to child.

The signup form gains a parent/guardian tick that reveals a block per child,
with the account-signup questions asked per child rather than per guardian
— they describe the student, not the account holder. Signup policies are
recorded once per child with the guardian as the acceptor, which is the
record that actually means something. A family that half-creates is rolled
back entirely rather than leaving a guardian who cannot re-register.

The booking and enrolment forms gain a "Who is this for?" picker listing
children first, so the default selection is never the parent — booking for
the wrong child is correctable, quietly billing a parent for their kid's
lesson is not. POST /bookings and POST /enrollments take an optional
student_id honoured only for that child's guardian; anything else is a 403.
That check is the authorisation boundary of the feature.

Payments and credits gain a payer: the charge names the child it was for and
the guardian who owes it, so per-child reporting is unchanged while notices,
receipts and the payment step reach the parent. Credit is held by the payer,
so one child's cancellation can settle a sibling's charge, and the daily
billing scan sends a guardian one notice covering every child.

Closes #132

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

assets/css/frontend.css

@@ -388,6 +388,108 @@
     }
 }
 
+/*
+ * "Who is this for?" picker — booking and enrolment. Present only on an account
+ * that books for more than one person, so it is styled as a normal field rather
+ * than a callout.
+ */
+.us-student-picker select {
+    max-width: 100%;
+}
+
+/* Whose lesson a row in the upcoming panel is — only shown on a family account. */
+.us-my-lesson-who {
+    font-weight: normal;
+    opacity: 0.75;
+}
+
+/* Parent/guardian signup: the child blocks revealed by the checkbox. */
+.us-guardian {
+    margin: 16px 0;
+    padding: 12px 14px;
+    border: 1px solid #ddd;
+    border-radius: 4px;
+}
+
+.us-guardian legend {
+    padding: 0 6px;
+    font-weight: 600;
+}
+
+.us-children-intro {
+    margin-top: 0;
+    font-size: 0.9em;
+    opacity: 0.8;
+}
+
+/*
+ * Each child is a bordered group so a family of three does not read as one long
+ * undifferentiated column of fields.
+ */
+.us-child {
+    margin-bottom: 12px;
+    padding: 10px 12px;
+    border-left: 3px solid #ddd;
+    background: #fafafa;
+}
+
+.us-child > p:last-child {
+    margin-bottom: 0;
+}
+
+/* The guardian's manage-children screen ([us_family]). */
+.us-family-list {
+    margin: 0 0 20px;
+    padding: 0;
+    list-style: none;
+}
+
+.us-family-child {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 8px 12px;
+    align-items: baseline;
+    padding: 10px 0;
+    border-bottom: 1px solid #eee;
+}
+
+.us-family-child-name {
+    font-weight: 600;
+}
+
+.us-family-child-dob {
+    font-size: 0.9em;
+    opacity: 0.75;
+}
+
+/*
+ * The actions sit at the far end of the row. Remove is its own form (it posts),
+ * so it is forced inline rather than taking a block of its own.
+ */
+.us-family-child-actions {
+    display: flex;
+    gap: 10px;
+    align-items: baseline;
+    margin-left: auto;
+}
+
+.us-family-remove {
+    display: inline;
+}
+
+/* The editing row replaces the child's line, so it spans the whole width. */
+.us-family-edit {
+    width: 100%;
+}
+
+@media (max-width: 640px) {
+    /* A name, a date and two actions do not fit one narrow line. */
+    .us-family-child-actions {
+        margin-left: 0;
+        width: 100%;
+    }
+}
+
 /* Shown only in block-editor previews (see BlockPreview). */
 .us-editor-note {
     font-size: 0.85em;