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]>

tests/Unit/Payment/CreditRepositoryTest.php

@@ -42,7 +42,7 @@ class CreditRepositoryTest extends TestCase
             );
         $this->db->insert_id = 300;
 
-        $credit = new Credit(5, 33.0, 33.0, 'CAD', 12, 77, 'Credit for cancelled lesson #77');
+        $credit = new Credit(5, 33.0, 33.0, currency: 'CAD', sourcePaymentId: 12, sourceLessonId: 77, reason: 'Credit for cancelled lesson #77');
         self::assertSame(300, $this->repo->insert($credit));
     }
 
@@ -108,4 +108,67 @@ class CreditRepositoryTest extends TestCase
 
         $this->repo->consume(5, 0.0);
     }
+
+    /**
+     * The balance is keyed on the payer, not the student, so a guardian's
+     * account carries the credits every one of their children earned.
+     */
+    public function testAvailableBalanceQueriesThePayer(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::on(static fn (string $sql): bool => str_contains($sql, 'payer_id = %d')), 'wp_us_credits', 5, Credit::STATUS_AVAILABLE)
+            ->andReturn('sql');
+        $this->db->shouldReceive('get_var')->once()->with('sql')->andReturn('60.00');
+
+        self::assertSame(60.0, $this->repo->availableBalance(5));
+    }
+
+    public function testFindAvailableByPayerReturnsCreditsOldestFirst(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::on(static fn (string $sql): bool => str_contains($sql, 'payer_id = %d')), 'wp_us_credits', 5, Credit::STATUS_AVAILABLE)
+            ->andReturn('sql');
+        $this->db->shouldReceive('get_results')->once()->with('sql')->andReturn([
+            (object) ['id' => '1', 'student_id' => '42', 'payer_id' => '5', 'amount' => '10.00', 'remaining' => '10.00', 'currency' => 'CAD', 'source_payment_id' => null, 'source_lesson_id' => null, 'reason' => null, 'status' => Credit::STATUS_AVAILABLE, 'created_at' => '2026-07-01 09:00:00', 'updated_at' => null],
+        ]);
+
+        $credits = $this->repo->findAvailableByPayer(5);
+
+        self::assertCount(1, $credits);
+        self::assertSame(42, $credits[0]->studentId);
+        self::assertSame(5, $credits[0]->payerId);
+    }
+
+    /**
+     * Rows written before guardian accounts existed carry payer_id 0; the
+     * installer points them at the student who was always the payer.
+     */
+    public function testBackfillPayerIdsPointsLegacyRowsAtTheStudent(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with('UPDATE %i SET payer_id = student_id WHERE payer_id = 0', 'wp_us_credits')
+            ->andReturn('sql');
+        $this->db->shouldReceive('query')->once()->with('sql');
+
+        $this->repo->backfillPayerIds();
+    }
+
+    public function testInsertDefaultsThePayerToTheStudent(): void
+    {
+        Functions\when('current_time')->justReturn('2026-06-08 12:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_credits',
+                Mockery::on(static fn (array $d): bool => $d['student_id'] === 5 && $d['payer_id'] === 5),
+                Mockery::type('array')
+            );
+        $this->db->insert_id = 301;
+
+        self::assertSame(301, $this->repo->insert(new Credit(5, 10.0, 10.0)));
+    }
 }