Add HST/tax support and payment reporting with HST aggregation

Studio Settings gains a default HST rate; the rate is frozen onto each
payment at booking and computed against the pre-tax subtotal, with the
total billed as subtotal + tax. The rate is overridable per booking on
My Lessons while unpaid (recomputing the tax amount), comped
registrations are never taxed, and receipts break out subtotal/HST/total.

Builds the payments report (roadmap #8) from us_payments: a monthly
per-instructor view with subtotal, HST collected, and grand-total
aggregation, plus a nonce-protected CSV export via admin-post. Studio
admins see all instructors and can filter; instructors are scoped to
their own rows. The Payment Report menu is gated on export_payments so
instructors (who lack manage_billing) can reach it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/Payment/PaymentRepositoryTest.php

@@ -62,6 +62,42 @@ class PaymentRepositoryTest extends TestCase
         self::assertTrue($this->repo->markPaid(50, 'USC-50'));
     }
 
+    public function testUpdateTaxRecomputesAmountFromRate(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/tax_amount = ROUND\( amount \* %f \/ 100, 2 \)/'), 13.0, 13.0, 50)
+            ->andReturn('UPDATE ...');
+
+        $this->db->shouldReceive('query')->once()->with('UPDATE ...')->andReturn(1);
+
+        self::assertTrue($this->repo->updateTax(50, 13.0));
+    }
+
+    public function testFindPaidBetweenFiltersByInstructor(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/status = %s AND paid_at >= %s AND paid_at < %s AND instructor_id = %d/'), ['paid', '2026-06-01 00:00:00', '2026-07-01 00:00:00', 3])
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_results')->andReturn([$this->row()]);
+
+        self::assertCount(1, $this->repo->findPaidBetween('2026-06-01 00:00:00', '2026-07-01 00:00:00', 3));
+    }
+
+    public function testFindPaidBetweenWithoutInstructorOmitsFilter(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::on(static fn (string $sql): bool => ! str_contains($sql, 'instructor_id')), ['paid', '2026-06-01 00:00:00', '2026-07-01 00:00:00'])
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_results')->andReturn([]);
+
+        self::assertCount(0, $this->repo->findPaidBetween('2026-06-01 00:00:00', '2026-07-01 00:00:00'));
+    }
+
     public function testFindByRegistrationReturnsPayment(): void
     {
         $this->db->shouldReceive('prepare')
@@ -98,6 +134,8 @@ class PaymentRepositoryTest extends TestCase
             'currency'                 => 'CAD',
             'method'                   => Payment::METHOD_ETRANSFER,
             'status'                   => Payment::STATUS_PENDING,
+            'tax_rate'                 => '0.00',
+            'tax_amount'               => '0.00',
             'etransfer_email'          => null,
             'stripe_payment_intent_id' => null,
             'receipt_number'           => null,