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/Payment/CreditRepository.php

@@ -16,6 +16,7 @@ class CreditRepository {
 			$this->table,
 			[
 				'student_id'        => $credit->studentId,
+				'payer_id'          => $credit->payerOrStudent(),
 				'amount'            => $credit->amount,
 				'remaining'         => $credit->remaining,
 				'currency'          => $credit->currency,
@@ -25,7 +26,7 @@ class CreditRepository {
 				'status'            => $credit->status,
 				'created_at'        => current_time( 'mysql' ),
 			],
-			[ '%d', '%f', '%f', '%s', '%d', '%d', '%s', '%s', '%s' ]
+			[ '%d', '%d', '%f', '%f', '%s', '%d', '%d', '%s', '%s', '%s' ]
 		);
 
 		return $this->db->insert_id;
@@ -56,15 +57,16 @@ class CreditRepository {
 	}
 
 	/**
-	 * A student's total unused credit balance (sum of the remaining amounts of every
-	 * still-available credit).
+	 * A payer's total unused credit balance (sum of the remaining amounts of every
+	 * still-available credit). Keyed on the payer, so a guardian's balance covers
+	 * credits earned by any of their children — one family, one balance.
 	 */
-	public function availableBalance( int $studentId ): float {
+	public function availableBalance( int $payerId ): float {
 		$total = $this->db->get_var(
 			$this->db->prepare(
-				'SELECT COALESCE( SUM( remaining ), 0 ) FROM %i WHERE student_id = %d AND status = %s',
+				'SELECT COALESCE( SUM( remaining ), 0 ) FROM %i WHERE payer_id = %d AND status = %s',
 				$this->table,
-				$studentId,
+				$payerId,
 				Credit::STATUS_AVAILABLE
 			)
 		);
@@ -73,17 +75,17 @@ class CreditRepository {
 	}
 
 	/**
-	 * A student's still-available credits, oldest first — the FIFO order they are
+	 * A payer's still-available credits, oldest first — the FIFO order they are
 	 * consumed in.
 	 *
 	 * @return list<Credit>
 	 */
-	public function findAvailableByStudent( int $studentId ): array {
+	public function findAvailableByPayer( int $payerId ): array {
 		$rows = $this->db->get_results(
 			$this->db->prepare(
-				'SELECT * FROM %i WHERE student_id = %d AND status = %s AND remaining > 0 ORDER BY created_at ASC, id ASC',
+				'SELECT * FROM %i WHERE payer_id = %d AND status = %s AND remaining > 0 ORDER BY created_at ASC, id ASC',
 				$this->table,
-				$studentId,
+				$payerId,
 				Credit::STATUS_AVAILABLE
 			)
 		);
@@ -92,7 +94,10 @@ class CreditRepository {
 	}
 
 	/**
-	 * Every credit for a student, newest first (admin history).
+	 * Every credit earned by a student, newest first — the admin history on their
+	 * own screen. Unlike the balance this is keyed on the student, so a child's
+	 * screen shows the credits their cancellations produced even though the
+	 * balance itself sits with their guardian.
 	 *
 	 * @return list<Credit>
 	 */
@@ -109,17 +114,30 @@ class CreditRepository {
 	}
 
 	/**
-	 * Draw down a student's credit balance by $amount, consuming their available
+	 * Backfill `payer_id` on credits written before guardian accounts existed,
+	 * where the student was always the payer. Run once from the installer so the
+	 * payer-keyed balance queries see those rows.
+	 */
+	public function backfillPayerIds(): void {
+		$sql = $this->db->prepare( 'UPDATE %i SET payer_id = student_id WHERE payer_id = 0', $this->table );
+
+		if ( null !== $sql ) {
+			$this->db->query( $sql );
+		}
+	}
+
+	/**
+	 * Draw down a payer's credit balance by $amount, consuming their available
 	 * credits oldest first and marking each fully-spent credit `consumed`. Stops once
 	 * the amount is exhausted; a balance shorter than $amount simply drains to zero.
 	 */
-	public function consume( int $studentId, float $amount ): void {
+	public function consume( int $payerId, float $amount ): void {
 		$remaining = round( $amount, 2 );
 		if ( $remaining <= 0.0 ) {
 			return;
 		}
 
-		foreach ( $this->findAvailableByStudent( $studentId ) as $credit ) {
+		foreach ( $this->findAvailableByPayer( $payerId ) as $credit ) {
 			if ( $remaining <= 0.0 ) {
 				break;
 			}