Fix payment review round: till integrity, HTTP client tests, concurrency tests, card-selection consolidation
Addresses the payment review (all 10 blocking + 2 minor findings):
Till money-integrity (CreateTillSale):
- Add pg_advisory_lock on the idempotency key (concurrent same-key double-funding race)
- Guard amount on pending-reuse retry (mirrors tip/gift-card guards)
- Explicitly complete the row for cash/on_the_house pending-reuse
- Reject method-switch on a live card-machine checkout (double-charge guard)
- 3 regression tests (amount-mismatch, cash-completes-row, method-switch)
BookingFlow:
- Fetch saved cards at the deposit step (was dead code)
- Charge the server-computed deposit_amount, not the client estimate
HTTP client tests (was untested): doJSON error parsing, refund sentinel
classification, payment/refund/card wire shapes, checkout polling states,
list-refunds pagination + 20-page guard, sha256 card idempotency key
Concurrency regression tests: real two-goroutine races for BuyGiftCard,
tip, and booking-payment locks asserting exactly-one record each
Frontend:
- Fix CRIT-1: zero-saved-card users blocked (all flows now handle it)
- Consolidate tip/deposit/Buy-Gift-Card card UI onto CardSelection
- Explicit save-card consent checkbox (was silent/inconsistent)
- Fix stale saved-card field names in BookingFlow (last4 -> last_4)
- Unique instance ids (crypto.randomUUID) in CardSelection/SquareCardInput
- UserPaymentModal: keep card form mounted on error + Try Again button
Health/docs: /api/health reports square state (mock/ok, was not_implemented),
close P1 backlog, correct stale webhook and env-var claims
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