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]>

src/Schema.php

@@ -138,6 +138,7 @@ class Schema {
                 id                BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 policy_version_id BIGINT UNSIGNED NOT NULL,
                 student_id        BIGINT UNSIGNED NOT NULL,
+                accepted_by       BIGINT UNSIGNED NOT NULL DEFAULT 0,
                 registration_type VARCHAR(20)     NOT NULL,
                 registration_id   BIGINT UNSIGNED NOT NULL,
                 accepted_at       DATETIME        NOT NULL,
@@ -145,12 +146,14 @@ class Schema {
                 PRIMARY KEY (id),
                 KEY policy_version_id (policy_version_id),
                 KEY student_id (student_id),
+                KEY accepted_by (accepted_by),
                 KEY registration (registration_type, registration_id)
             ) {$charset};",
 
 			"CREATE TABLE {$prefix}us_payments (
                 id                       BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 student_id               BIGINT UNSIGNED NOT NULL,
+                payer_id                 BIGINT UNSIGNED NOT NULL DEFAULT 0,
                 instructor_id            BIGINT UNSIGNED NOT NULL,
                 registration_type        VARCHAR(20)     NOT NULL,
                 registration_id          BIGINT UNSIGNED NOT NULL,
@@ -172,6 +175,7 @@ class Schema {
                 paid_at                  DATETIME        DEFAULT NULL,
                 PRIMARY KEY (id),
                 KEY student_id (student_id),
+                KEY payer_id (payer_id),
                 KEY instructor_id (instructor_id),
                 KEY registration (registration_type, registration_id),
                 KEY status (status)
@@ -180,6 +184,7 @@ class Schema {
 			"CREATE TABLE {$prefix}us_credits (
                 id                BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 student_id        BIGINT UNSIGNED NOT NULL,
+                payer_id          BIGINT UNSIGNED NOT NULL DEFAULT 0,
                 amount            DECIMAL(10,2)   NOT NULL DEFAULT 0,
                 remaining         DECIMAL(10,2)   NOT NULL DEFAULT 0,
                 currency          VARCHAR(3)      NOT NULL DEFAULT 'CAD',
@@ -191,6 +196,7 @@ class Schema {
                 updated_at        DATETIME        DEFAULT NULL,
                 PRIMARY KEY (id),
                 KEY student_id (student_id),
+                KEY payer_id (payer_id),
                 KEY status (status),
                 KEY source_lesson_id (source_lesson_id)
             ) {$charset};",
@@ -229,6 +235,21 @@ class Schema {
                 KEY status (status)
             ) {$charset};",
 
+			// Links a parent/guardian account to a child who books through it. The
+			// child is a real (login-less) wp_users row, so student_id keeps meaning
+			// "a WordPress user" on every other table.
+			"CREATE TABLE {$prefix}us_guardians (
+                id           BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                guardian_id  BIGINT UNSIGNED NOT NULL,
+                student_id   BIGINT UNSIGNED NOT NULL,
+                relationship VARCHAR(50)     NOT NULL DEFAULT '',
+                created_at   DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                UNIQUE KEY guardian_student (guardian_id, student_id),
+                KEY guardian_id (guardian_id),
+                KEY student_id (student_id)
+            ) {$charset};",
+
 			"CREATE TABLE {$prefix}us_group_access (
                 id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 offering_id   BIGINT UNSIGNED NOT NULL,