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/Guardian/ChildLoginGate.php

@@ -0,0 +1,62 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Guardian;
+
+use Unsupervised\Schedular\Auth\RoleManager;
+
+/**
+ * Keeps child accounts unusable as logins. A child holds the `us_student` role
+ * so every `student_id` lookup in the schema keeps working, but nobody is ever
+ * given its credentials — this closes the door the role would otherwise leave
+ * open:
+ *
+ * - authentication is refused outright, and
+ * - the booking capability is withheld, so nothing that reaches a capability
+ *   check on a child's own session (there should be none) can book as them.
+ *
+ * Both key off the `us_child` meta, so ordinary students are untouched.
+ */
+class ChildLoginGate {
+
+	public function register(): void {
+		add_filter( 'wp_authenticate_user', [ $this, 'blockChildLogin' ], 10, 1 );
+		add_filter( 'user_has_cap', [ $this, 'withholdBooking' ], 10, 4 );
+	}
+
+	/**
+	 * Refuse authentication for a child account. Runs after password
+	 * verification, so it holds even if a password were somehow set on one.
+	 *
+	 * @param \WP_User|\WP_Error $user Authenticating user, or an earlier error.
+	 * @return \WP_User|\WP_Error
+	 */
+	public function blockChildLogin( $user ) {
+		if ( $user instanceof \WP_User && GuardianService::isChild( (int) $user->ID ) ) {
+			return new \WP_Error(
+				'us_child_account',
+				esc_html__( 'This is a child account and cannot be signed in to. Please sign in with the parent or guardian account.', 'unsupervised-schedular' )
+			);
+		}
+
+		return $user;
+	}
+
+	/**
+	 * Strip the booking capability from a child account, so the only route to a
+	 * lesson in their name is their guardian's authorised booking.
+	 *
+	 * @param array<string, bool> $allcaps All capabilities currently held.
+	 * @param array<int, string>  $caps    Required capabilities (unused).
+	 * @param array<int, mixed>   $args    Callback args (unused).
+	 * @param mixed               $user    The user being checked (a WP_User in practice).
+	 * @return array<string, bool>
+	 */
+	public function withholdBooking( array $allcaps, array $caps, array $args, mixed $user ): array {
+		if ( $user instanceof \WP_User && GuardianService::isChild( (int) $user->ID ) ) {
+			unset( $allcaps[ RoleManager::CAP_BOOK_LESSON ] );
+		}
+
+		return $allcaps;
+	}
+}