fix: review round 6 — P0 deposit charge, idempotency rotation, dev-safety guard, 2FA/webhook hardening

Sixth fresh-eyes review pass (5 agents: goal, QA, code-quality, security,
context-mining). QA FAILED the deposit-required new-card flow; the P0 root
cause was backend + frontend, now fixed. All 20 packages green.

P0 money-safety:
- Deposit-required bookings now actually charge the deposit on new-card
  payment. Two-part fix: (1) CreateBookingHandler re-reads the
  trigger-maintained total_amount/total_duration_minutes from the DB after the
  booking_services insert (the INSERT..RETURNING row predates the recalc
  trigger, so TotalAmount serialized as 0 and DepositPaid computed TRUE on an
  unpaid booking — the frontend gate trusted deposit_paid:true, never charged,
  and confirmed the booking with zero payment rows); (2) BookingFlow.svelte
  gates the confirmation view on depositPaid and guards against re-creating a
  booking on retry. Regression test
  TestBookings_Create_DepositPaidFalseOnUnpaidBooking.

Payments (idempotency + money):
- deriveBookingPaymentIdempotencyKey: no-client-key fallback now advances a
  sequence for repeatable types (partial) and rotates past refunded completed
  rows, so refund-then-repay and equal-amount partials diverge onto distinct
  keys; an un-refunded completed row keeps its key (double-charge protection
  holds). Dedup hits on refunded rows now 409, never stale success.
- chargeFailureStatus default is 503 (ambiguous), never 402; table test.
- Flaky TestBookingPayment_FullPayment_SplitsIntoDepositAndBalance fixed
  (ORDER BY payment_type).
- resolveChargeSource: orphaned card-on-file disabled via DeleteCardOnFile
  when SaveCardForUser fails (best-effort, redacted log); retry path preserved.

Square client:
- Dev builds HARD-FAIL (panic) on SQUARE_ENVIRONMENT=production without
  SQUARE_ALLOW_REAL_API=1; sandbox routes with a loud banner.
- Mock fault-injection FailAfterCommit (commit-then-5xx) exercises the exact
  lost-response same-key retry; SimulateCardTokenUsed; 45-char idempotency-key
  cap parity; SquareEnvironment/SquareLocationID shared env helpers used by
  the sweep (env contract no longer comment-only).
- listRefunds truncation now errors (money-sensitive reconcile retries
  instead of over-refunding); getCardsOnFile truncation loudly logged.

Webhooks + 2FA:
- square-environment header checked fail-closed (403) when configured env is
  production/sandbox; dispatch DB work bounded by 30s timeout contexts.
- 2FA codes HMAC-SHA256 pepper'd (TWO_FACTOR_PEPPER) with legacy-hash
  migration + upgrade-on-verify; disable-flow mint cooldown (1/min, 429) caps
  the brute-force loop; in-lockout records never LRU-evicted.

Repo hygiene:
- env-docs CI gate green again (FRONTEND_ORIGIN + SQUARE_ALLOW_REAL_API +
  TWO_FACTOR_PEPPER documented; Vite DEV built-in allowlisted).
- Dead square_deposits schema dropped; obsidian/README/legal-page drift fixed
  (consumeradvice.scot signposting, CORS allowlist, p11 R3/P13, T1).
- 2FA disable residual documented; P6 email/SMS delivery and P12 sandbox
  smoke test remain the pre-go-live gates.

Verification: go test -tags test,dev -count=1 -parallel 8 ./... (20/20 ok),
go build ./... + -tags dev, go vet clean, svelte-check 0 errors, env-docs
gate OK, live deposit-required flow re-verified end-to-end (deposit £11
charged, square_payment_id recorded).

.env.example

@@ -41,6 +41,13 @@ SQUARE_ACCESS_TOKEN=
 SQUARE_LOCATION_ID=
 SQUARE_TERMINAL_DEVICE_ID=
 SQUARE_ENVIRONMENT=mock
+# SQUARE_ALLOW_REAL_API — dev-build safety valve. In a `//go:build dev` build the
+# backend HARD-FAILS (refuses to construct the client) when SQUARE_ENVIRONMENT
+# is 'production', because a leftover/typo'd production env + real key in a dev
+# shell would create real charges. Set SQUARE_ALLOW_REAL_API=1 ONLY to
+# deliberately route a dev build to the real production API. Never set in a
+# deployed production build.
+SQUARE_ALLOW_REAL_API=
 # 2FA (PSD2 SCA stand-in) for online card payments. Enforcement is FAIL-CLOSED:
 # ON unless REQUIRE_2FA explicitly disables it (false/0/off/no, case-insensitive)
 # OR SQUARE_ENVIRONMENT explicitly equals one of mock/dev/development/test.
@@ -53,6 +60,9 @@ SQUARE_ENVIRONMENT=mock
 # environments an operator must relay the logged code to the user out-of-band;
 # the API never returns the code while enforcement is ON.
 REQUIRE_2FA=true
+# TWO_FACTOR_PEPPER — server-side pepper for HMAC-hashing 2FA codes (optional
+# pre-launch; if unset, codes are hashed without pepper and a warning is logged)
+TWO_FACTOR_PEPPER=
 # Webhook config MUST exactly match the Square Dashboard webhook subscription
 # (URL + signature key). If SQUARE_WEBHOOK_NOTIFICATION_URL is left unset it
 # defaults to http://localhost:8080/webhooks/square, which is fail-closed (503
@@ -92,6 +102,11 @@ NO_COLOR=
 # Frontend
 VITE_BACKEND_URL=http://localhost:8080
 
+# Backend CORS allowlist — comma-separated list of allowed frontend origins
+# (read by the backend CORS middleware, see backend/main.go). Falls back to
+# http://localhost:5173 when unset.
+FRONTEND_ORIGIN=http://localhost:5173
+
 # Local S3 (Rustfs) — requires GO_TESTING=1 or dev build tag
 # These are dev-only overrides used by the dev S3 implementation
 RUSTFS_ENDPOINT=http://rustfs:9000