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>

src/Payment/Payment.php

@@ -40,6 +40,8 @@ class Payment {
 		public readonly string $currency = 'CAD',
 		public readonly string $method = self::METHOD_ETRANSFER,
 		public readonly string $status = self::STATUS_PENDING,
+		public readonly float $taxRate = 0.0,
+		public readonly float $taxAmount = 0.0,
 		public readonly ?string $etransferEmail = null,
 		public readonly ?string $stripePaymentIntentId = null,
 		public readonly ?string $receiptNumber = null,
@@ -58,6 +60,8 @@ class Payment {
 			currency:              $row->currency,
 			method:                $row->method,
 			status:                $row->status,
+			taxRate:               (float) $row->tax_rate,
+			taxAmount:             (float) $row->tax_amount,
 			etransferEmail:        $row->etransfer_email,
 			stripePaymentIntentId: $row->stripe_payment_intent_id,
 			receiptNumber:         $row->receipt_number,
@@ -71,6 +75,13 @@ class Payment {
 		return self::STATUS_PAID === $this->status;
 	}
 
+	/**
+	 * Amount billed including tax.
+	 */
+	public function total(): float {
+		return round( $this->amount + $this->taxAmount, 2 );
+	}
+
 	/**
 	 * Returns a plain array representation of the payment.
 	 *
@@ -85,6 +96,9 @@ class Payment {
 			'etransfer_email'   => $this->etransferEmail,
 			'registration_id'   => $this->registrationId,
 			'amount'            => $this->amount,
+			'tax_rate'          => $this->taxRate,
+			'tax_amount'        => $this->taxAmount,
+			'total'             => $this->total(),
 			'currency'          => $this->currency,
 			'method'            => $this->method,
 			'status'            => $this->status,