Add e-transfer destination email (studio default + offering/booking overrides)

The e-transfer destination is resolved at booking time (offering override ->
studio default) and frozen onto the payment, so each record keeps where the
student was directed. It can then be corrected per booking.

- StudioSettings: us_etransfer_email option + a Default e-transfer email field
  on the Studio Settings page.
- Offering: etransfer_email column/field (instructor override) across VO, repo,
  REST endpoint, admin controller, and form.
- Payment: etransfer_email column on the payment (frozen record) +
  PaymentRepository::updateEtransferEmail; PaymentService freezes it from the
  offering override or studio default at creation; booking/enrolment pass the
  offering override.
- My Lessons: instructors edit the e-transfer email per pending lesson payment
  (ownership-checked).
- Payments queue: studio admin can correct the email at confirmation (for when
  a student sends it to the wrong place).
- Docs updated.

Tests: Payment/Offering rows + PaymentService freezing. composer test (148),
cs, and PHPStan level 6 all pass.

Refs #7

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

docs/features/payments.md

@@ -41,6 +41,18 @@ default applies — `card` if Stripe is configured, otherwise `etransfer`
 | `etransfer`| Payment row created `pending`; admin marks it `paid` when funds arrive |
 | `comp`     | No charge; registration is confirmed immediately, no payment row required |
 
+## E-transfer Destination Email
+Where students send e-transfers is resolved and **frozen onto the payment** at
+booking time (`us_payments.etransfer_email`), so each record keeps the destination
+the student was given. Resolution at creation:
+
+1. **Offering override** — `us_offerings.etransfer_email`, set by the instructor on the offering.
+2. **Studio default** — the `us_etransfer_email` option (Studio Settings, `manage_billing`).
+
+After booking, the destination on a payment can be corrected per booking:
+- **My Lessons** — the instructor edits the e-transfer email for a pending lesson payment.
+- **Payments queue** — when marking an e-transfer received, the studio admin can update the email it was actually sent to before confirming.
+
 ## Data Model — `{prefix}us_payments`
 
 | Column                     | Type             | Notes                                                  |
@@ -54,6 +66,7 @@ default applies — `card` if Stripe is configured, otherwise `etransfer`
 | `currency`                 | VARCHAR(3)       | ISO 4217, e.g. `CAD`                                  |
 | `method`                   | VARCHAR(20)      | `card` / `etransfer` / `comp`                         |
 | `status`                   | VARCHAR(20)      | `pending` / `paid` / `failed` / `refunded`            |
+| `etransfer_email`          | VARCHAR(191)     | Frozen e-transfer destination; editable until confirmed |
 | `stripe_payment_intent_id` | VARCHAR(255)     | Stripe PaymentIntent id; NULL for e-transfer / comp   |
 | `receipt_number`           | VARCHAR(50)      | Sequential receipt id; set when `paid`                |
 | `receipt_sent_at`          | DATETIME         | When the receipt email was sent; NULL until sent      |