Apply second-round review fixes: idempotency-key length caps, stable-sentinel card keys, test-isolation, naming

Money-safety idempotency hardening (I1, wide):
- validate:"max=45" on CreateTerminalPayment/BookingPayment/Refund/Tip/
  BuyGiftCard idempotency keys (all feed Square's 45-char /v2/payments,
  /v2/refunds, /v2/cards caps); BuyGiftCard corrected from a wrongly-loose
  max=64. Till keeps max=64 (its key also feeds the 64-char terminal-checkout
  endpoint).
- Explicit 45-char guard in RefundPayment: the one handler that decodes
  RefundRequest without running the struct validator, so the tag alone was
  inert; a longer key would 400 at Square and be misclassified as a
  definitive refund decline.
- New TestIdempotencyKey_OverLength_RejectedAcrossPaymentHandlers covers all
  six endpoints (terminal saved-card, booking, tip, gift-card, till, refund).

Stable-sentinel card identity in idempotency keys (C1, wide):
- BookingFlow deposit key now uses the 'new-card' sentinel instead of
  embedding the cnon: nonce (matches UserPaymentModal/account). A re-tokenize
  after a spent nonce no longer regenerates the key, closing a lost-response
  double-charge window.
- TipPayment + UserBookingModal tip keys now include card identity
  (selectedCardId || 'new-card'); previously keyed on amount only, so a
  same-amount tip on a DIFFERENT card reused the key and deduped a distinct
  charge. Resets cleared in every success/close path.

Test isolation (R1): TestRefund_PendingResume_NewKeyAfterModalReopen no
longer t.Parallel — it swaps the package-level SquareClient mid-test and a
concurrent parallel test could observe the swapped instance.

Naming/quality (M1/M2/M4): resolveChargeSource local renamed savedRowID (was
shadowing the cardID *string parameter); BuyGiftCard fallback prefix
"till-" -> "gc-"; saved-card terminal response key "checkout_id" -> "payment_id"
(it holds a DB payment row, not a Square checkout) with matching frontend
fallback. README maintenance-job count corrected 24 -> 25.

Full suite 25/25 + race clean via run-tests.sh lockfile; svelte-check 0
errors/warnings; production build succeeds.

README.md

@@ -4,7 +4,7 @@ Nail salon booking platform — Go 1.26.5 backend + SvelteKit 5 SPA + PostgreSQL
 
 ## Features
 
-**Booking**: Self-service (customer), walk-in (admin), call-in (admin). Slot reservations prevent double-booking (4 TTL types). **Self-blocking prevention**: `excludeUserID` parameter filters a user's own `RESERVATION` entries from time blocker overlap checks, allowing re-reservation and booking at overlapping slots. **Explicit cancellation**: `DELETE /api/bookings/reserve` releases a user reservation; `DELETE /api/admin/bookings/reserve` releases an admin walk-in/call-in reservation. **Background cleanup**: Centralised cron scheduler (`backend/internal/jobs/`) runs 24 maintenance jobs: reservation/deposit cleanup every 5min, hourly campaign transitions, daily unpaid-booking notifications, staged default hours auto-apply, GDPR anonymization, financial aggregation, and token/code cleanup. Guest accounts with GDPR-compliant anonymization (including `RESERVATION:edit_request:%` scrubbing). Service eligibility based on age + patch test validity. Overlap checks use `FOR UPDATE` row locks inside transactions. Closing-hours validation (`closing_time.go`) resolves both current and staged default hours.
+**Booking**: Self-service (customer), walk-in (admin), call-in (admin). Slot reservations prevent double-booking (4 TTL types). **Self-blocking prevention**: `excludeUserID` parameter filters a user's own `RESERVATION` entries from time blocker overlap checks, allowing re-reservation and booking at overlapping slots. **Explicit cancellation**: `DELETE /api/bookings/reserve` releases a user reservation; `DELETE /api/admin/bookings/reserve` releases an admin walk-in/call-in reservation. **Background cleanup**: Centralised cron scheduler (`backend/internal/jobs/`) runs 25 maintenance jobs: reservation/deposit cleanup every 5min, hourly campaign transitions, daily unpaid-booking notifications, staged default hours auto-apply, GDPR anonymization, financial aggregation, and token/code cleanup. Guest accounts with GDPR-compliant anonymization (including `RESERVATION:edit_request:%` scrubbing). Service eligibility based on age + patch test validity. Overlap checks use `FOR UPDATE` row locks inside transactions. Closing-hours validation (`closing_time.go`) resolves both current and staged default hours.
 
 **Payments**: Square Terminal (in-person, via `CreateTerminalCheckout`) + online card payments via saved cards or new cards tokenized through the Square Web Payments SDK (`cnon:` nonces — new-card entry falls back to `CardEntryUnavailable` only when neither mock mode nor Square credentials are configured). The backend accepts only tokens, never raw PANs (PCI-DSS parity, mirrored in the dev mock). Cash till sales record the gift-card value and are marked completed, with no tendered/change fields. Any change or overpayment is handled manually by the admin at the counter. Gift cards (12-digit code or account balance). Saved cards for faster checkout. Tips on completed bookings. Refunds with notice-period tiers and deposit protection (72h/24h thresholds). All payment types: deposit, full, partial, balance, tip. Payment >20% of total promotes `pending_release` bookings back to `confirmed`. Deposit paid is computed from payments on-the-fly. The first 50% of each payment is always carved out as deposit (via `buildSplitRecords`); any overflow beyond the booking total becomes a tip. A bounded PostgreSQL advisory try-lock (`pg_try_advisory_lock`, ~30 × 100ms ≈ 3s bound) serializes payment attempts per-booking to prevent two-tab double-payment races. Gift card purchases insert a pending payment record with VAT before calling Square — the DB transaction commits first, so Square failures leave a retryable pending record (same-key retries reuse it). Three background sweeps close Square's ~24h idempotency-key retention window: `sweep-pending-square-refunds` reconciles/retries stuck refunds (with a 23h age guard), `sweep-stale-pending-payments` fails stale pending payments/till-sales so a late retry cannot issue a second charge, and `sweep-stale-terminal-checkouts` cancels card-machine checkouts still pending at Square after an hour so a never-polled checkout cannot complete into an invisible, untracked charge.