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>

README.md

@@ -35,8 +35,8 @@ model, REST API, classes, and tests. For contributor/architecture guidance see
 | Lesson booking (offering → questions → policies) | [lesson-booking.md](docs/features/lesson-booking.md) | ✅ Implemented |
 | Group classes (capacity-enforced enrolment) | [group-classes.md](docs/features/group-classes.md) | ✅ Implemented |
 | Student administration (studio-admin view) | [student-administration.md](docs/features/student-administration.md) | ✅ Implemented |
-| Payments (e-transfer/comp + receipts; Stripe card charge pending) | [payments.md](docs/features/payments.md) | 🟡 Partial |
-| Payment reporting (monthly per-instructor + CSV) | [payment-reporting.md](docs/features/payment-reporting.md) | 🟡 Planned |
+| Payments (e-transfer/comp + receipts + HST; Stripe card charge pending) | [payments.md](docs/features/payments.md) | 🟡 Partial |
+| Payment reporting (monthly per-instructor + HST + CSV) | [payment-reporting.md](docs/features/payment-reporting.md) | ✅ Implemented |
 
 > Payments are deliberately deferred to the end: booking and enrolment ship with a
 > clean seam (a lesson lands `pending`, an enrolment `active`, with `payment_id`