feat: SCA-only saved-card charges — 2FA charge fallback removed (C6), versioned consent fields, token provenance

PSR 2017 reg 100 makes SCA mandatory and non-waivable for customer-initiated
stored-credential charges; a merchant-side 2FA check cannot legally substitute
for it (authorising a token-less charge via 2FA leaves the MERCHANT liable for
ECI 7 / SLI 210 chargebacks and reg 77(6) compensation regardless of consent).

- payments/twofa.go: the homegrown 2FA fallback for token-less saved-card
  charges is REMOVED ENTIRELY. requireTwoFactorForCardAccess is now SCA-only:
  a non-empty Square verification_token (charge surfaces, token forwarded to
  Square) skips the gate; anything else is refused 402 verification_required.
  enforceSCAFallbackConsent is a compile-compatible no-op (fallback never runs).
- New requireTwoFactorForCardAccessWithTokenValidation distinguishes surfaces
  where the token IS forwarded to Square (charge — Square validates it) from
  card-SAVE surfaces (token client-asserted, never forwarded: a non-empty token
  must NOT skip the save gate, auth-F1).
- SCA tokenize-result wire contract (C1): a saved card charged with a fresh
  one-time tokenize-result sends the token as the charge SOURCE (new_card_token
  -> source_id) alongside saved_card_id, never a separate verification_token.
  resolveChargeSource resolves the saved-card branch FIRST (customer from the
  card row, token as source) so combined token+card requests are SCA-clean.
- C6 consent fields (consent_version / consent_accepted) added to the booking/
  tip/till/gift-card charge requests, enforced server-side before any fallback
  charge could reach Square and recorded on the 2fa_fallback_charge audit row;
  logVerificationTokenProvenance traces minted tokens to their charge.
- user 2FA issuance gate refactored into pure build-agnostic functions
  (twoFAPepperConfigured / twoFADeliveryChannelConfigured /
  twoFAEnsureIssueAllowedStrict) shared with the payments re-issue path and
  exercised directly by the test,dev suite; TWO_FACTOR_FALLBACK switch and
  .env.example entry removed; startup posture notes updated.
- Test coverage: fail-closed 2FA production gates (pepper/delivery), token
  validation on save vs charge surfaces, completion idempotency, idempotency
  key determinism, refund-policy 72h/24h epsilon boundaries, VAT parity.

backend/handlers/user/twofa_prod_variant_dev_test.go

@@ -0,0 +1,11 @@
+//go:build dev || test
+
+package user
+
+// twofaProdVariant reports whether the PRODUCTION 2FA issuance/delivery
+// functions (twofa_prod.go, !dev && !test) are the compiled ones in this
+// build. Under `dev` OR `test` tags the dev/test variants (twofa_dev.go) are
+// compiled instead — always-allowed issuance, trivially-true delivery — so
+// the prod fail-closed branches are unreachable there.
+//lint:ignore U1000 referenced only from the prod-tag test (twofa_prod_test.go, !dev && !test); deliberately unused under dev/test tags
+const twofaProdVariant = false