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 <[email protected]>

src/Payment/PaymentService.php

@@ -41,6 +41,10 @@ class PaymentService {
 			? $offeringEtransferEmail
 			: ( '' !== $this->settings->etransferEmail() ? $this->settings->etransferEmail() : null );
 
+		// HST is frozen from the studio default at booking; comped students are not taxed.
+		$taxRate   = Payment::METHOD_COMP === $method ? 0.0 : $this->settings->hstRate();
+		$taxAmount = round( $amount * $taxRate / 100, 2 );
+
 		$id = $this->payments->insert(
 			new Payment(
 				studentId:        $studentId,
@@ -51,6 +55,8 @@ class PaymentService {
 				currency:         $currency,
 				method:           $method,
 				status:           $status,
+				taxRate:          $taxRate,
+				taxAmount:        $taxAmount,
 				etransferEmail:   $etransferEmail,
 			)
 		);