Add payments foundation (e-transfer/comp, Stripe config, receipts)

Implements the payments foundation for #7. Without Stripe credentials
everything works on e-transfer (pending payment confirmed by a studio
admin); when Stripe keys are configured the default flips to credit card.
Per-student override (card/etransfer/comp) is set on the student detail.

- Schema: us_payments (amount DECIMAL dollars, method, status, receipt,
  stripe intent id).
- src/Payment/: Payment VO, PaymentRepository, StudioSettings (Stripe
  options + isStripeConfigured + settings page), BillingMethodResolver
  (per-student override; default card if configured else etransfer),
  ReceiptMailer, PaymentService (create at registration, link payment_id,
  comp->paid+confirm, markPaid->confirm+receipt), PaymentController
  (e-transfer confirmation queue), PaymentEndpoint (PATCH /payments/{id}).
- Booking + enrolment create the payment from the offering price; comp
  auto-confirms the lesson; setPaymentId on both repositories.
- Admin: Studio Settings + Payments menus (manage_billing); per-student
  billing method on the student detail page.
- Docs: payments.md + README updated.

Deferred to a follow-up: the live Stripe card charge (PaymentIntent +
Stripe.js Elements + webhook + stripe/stripe-php). Until then a card
payment is created pending and confirmed like an e-transfer.

Tests: tests/Unit/Payment/ (VO, repository, resolver, service, mailer).
composer test (147), cs, and PHPStan level 6 all pass.

Refs #7

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Schema.php

@@ -140,6 +140,28 @@ class Schema {
                 KEY registration (registration_type, registration_id)
             ) {$charset};",
 
+			"CREATE TABLE {$prefix}us_payments (
+                id                       BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                student_id               BIGINT UNSIGNED NOT NULL,
+                instructor_id            BIGINT UNSIGNED NOT NULL,
+                registration_type        VARCHAR(20)     NOT NULL,
+                registration_id          BIGINT UNSIGNED NOT NULL,
+                amount                   DECIMAL(10,2)   NOT NULL DEFAULT 0,
+                currency                 VARCHAR(3)      NOT NULL DEFAULT 'CAD',
+                method                   VARCHAR(20)     NOT NULL DEFAULT 'etransfer',
+                status                   VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                stripe_payment_intent_id VARCHAR(255)    DEFAULT NULL,
+                receipt_number           VARCHAR(50)     DEFAULT NULL,
+                receipt_sent_at          DATETIME        DEFAULT NULL,
+                created_at               DATETIME        NOT NULL,
+                paid_at                  DATETIME        DEFAULT NULL,
+                PRIMARY KEY (id),
+                KEY student_id (student_id),
+                KEY instructor_id (instructor_id),
+                KEY registration (registration_type, registration_id),
+                KEY status (status)
+            ) {$charset};",
+
 			"CREATE TABLE {$prefix}us_group_enrollments (
                 id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 offering_id   BIGINT UNSIGNED NOT NULL,