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 <[email protected]>

src/Schema.php

@@ -64,6 +64,7 @@ class Schema {
                 term_start       DATE            DEFAULT NULL,
                 term_end         DATE            DEFAULT NULL,
                 schedule_note    VARCHAR(191)    DEFAULT NULL,
+                etransfer_email  VARCHAR(191)    DEFAULT NULL,
                 is_active        TINYINT(1)      NOT NULL DEFAULT 1,
                 created_at       DATETIME        NOT NULL,
                 PRIMARY KEY (id),
@@ -150,6 +151,7 @@ class Schema {
                 currency                 VARCHAR(3)      NOT NULL DEFAULT 'CAD',
                 method                   VARCHAR(20)     NOT NULL DEFAULT 'etransfer',
                 status                   VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                etransfer_email          VARCHAR(191)    DEFAULT NULL,
                 stripe_payment_intent_id VARCHAR(255)    DEFAULT NULL,
                 receipt_number           VARCHAR(50)     DEFAULT NULL,
                 receipt_sent_at          DATETIME        DEFAULT NULL,