fix: review-loop A — discount credit on admin payments, campaign over-credit cap, sweep replay window, dedup refund revalidation, duplication/modularisation, GBP pence naming

Round-A fresh review (6 agents) + fix + secondary cross-cutting + verification rounds:
- F1: campaign discounts reduce the charged amount (deposit credit + admin PaymentModal discounted total); capDiscountToRemainingObligation prevents over-credit at completion in all four campaign blocks
- F2: sweep replay rescue distinguishes legitimate same-key retries (21h window) from expired-key new charges; ccof blind-fails leave pending + CRITICAL instead of clawing back
- F3: post-start online overflow carved as a tip record (mirrors terminal split builder)
- A1: single-source Square decline-code classification (till delegates to square.IsDefinitivePaymentError)
- A2/A5: refund attempt-cap literals consolidated; refund-failure counter capped + reset on terminal resolutions + admin notifications
- A3/A9: idempotency helpers adopted across derivations; IsExplicitDevOrMockEnv relocated + all gates unified (incl. health-check)
- A7: 2FA user+IP limiter + TRUST_PROXY_HEADERS startup warning; SNAPSHOT_ENC_KEY startup validation; TWO_FACTOR_PEPPER docs corrected
- A8: snapshot encryption on all 6 write sites + marker-aware reuse paths; MPV->SPV effective voucher type (single VAT point)
- A10/A11/A12/A16: gift-card slot scan advances past failed; amount-aware refund reconciliation; completed-booking refund re-check; PaymentWasRefunded on SquareClient interface
- Dedup refund revalidation on tip/terminal/gift-card paths; sweep acknowledged_at IS NULL parity; refund-notification single source (exported payments.InsertRefundFailedNotifications)
- Duplication/modularisation round: shared frontend helpers (sanitizeDecimalInput, campaignDiscountCents, twoFactorBlocksSavedCards getter, generateUUID), single-source MaxIdempotencyKeyLength, notification-helper consolidation, snapshot-guard comments
- Cross-cutting GBP rename: Cents->Pence across backend + frontend + tests (26 identifiers, 16 files)
- Tests: 11 behavior-change tests updated to new invariants; coverage for fixed functions; frontend vitest 55 tests; docs corrected (test counts, 2FA delivery, pre-launch checklist, resolution status)
- gitleaks: allowlist backend/internal/square test fixtures (mock idempotency keys)

All 25 backend packages pass; frontend 55/55 + build clean; env-docs 41/41.

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 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.
+**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 26 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.
 
@@ -62,6 +62,8 @@ docker compose up --build -d
 
 `SQUARE_ALLOW_REAL_API` is a dev-build safety valve: a `//go:build dev` build **HARD-FAILS** (panics) when `SQUARE_ENVIRONMENT=production` unless this is set to `1`, so a typo'd or leftover production value in a dev shell cannot create real charges. Sandbox is allowed in a dev build (with a loud banner). Never set it in a deployed production build.
 
+`TRUST_PROXY_HEADERS` (backend `.env`) defaults to `false`. The backend sits behind a trusted proxy in every real deployment — the nginx in `compose.yml` and/or the Cloudflare edge — which overwrites `X-Real-IP` / `CF-Connecting-IP` with the real client IP. Set `TRUST_PROXY_HEADERS=true` for those deployments: without it every per-IP rate-limit key collapses onto the proxy's IP, so any one client can exhaust the shared per-IP budget and throttle the whole surface for everyone (and per-IP limiter protection is effectively bypassed). Keep it `false` only when the backend is origin-exposed. `compose.yml` deliberately never sets it — the operator decides per deployment (the value is passed through the repo-root `.env` via `env_file`).
+
 | Service | URL |
 |---------|-----|
 | Frontend | http://localhost |
@@ -95,7 +97,7 @@ Default logins (password: `password`):
 ```bash
 cd backend && go build -o bin/backend ./main.go
 cd frontend && npm ci && npm run build
-cd backend && go test -tags "test,dev" -count=1 -parallel 8 ./...   # 2,269 tests compiled under the test,dev tags, as of 13 Aug 2026 (~2min)
+cd backend && go test -tags "test,dev" -count=1 -parallel 8 ./...   # 2,333 tests compiled under the test,dev tags, as of 14 Aug 2026 (~2min)
 cd backend && go test -tags "test,dev" -count=1 -race -timeout 480s ./...  # race detector (all packages, ~4min)
 # NOTE: -count=N>1 is unreliable for handlers/payments and handlers/webhooks —
 # those suites share package-global state (Square mock ledger, in-memory webhook