fix: review-loop hardening — identical-body replay, 2FA gates, webhook at-least-once, GDPR scrub

Follow-up to the comprehensive payment-system review. Fixes the issues the
review found in the initial integration, plus the rough edges it introduced.

Money-safety:
- Replay-by-key now replays the FULL original request verbatim from a stored
  square_request_snapshot, so a retained idempotency key returns the original
  payment instead of IDEMPOTENCY_KEY_REUSED (previously the row sat pending
  forever). IDEMPOTENCY_KEY_REUSED remains ambiguous (never proof of no charge).
- Dev mock mirrors real Square for unknown-key replays: ccof: saved-card
  sources are charged and rescued; spent cnon: nonces surface
  ErrReplayKeyNotRetained. (Fixes dev/prod parity divergence.)
- Webhook dedup row committed AFTER dispatch (at-least-once); FAILED till sales
  claw back gift-card funding; event-type strings match Square's real catalog.
- Expired-gift-card cancellation refunds set creditFailed (never a phantom
  'completed' refund); cancellation refunds lock all payment rows ascending.
- Sweep never rescue-completes a gift-card purchase without delivering the card.
- Tip no-client-key fallback is a deterministic count-based key under the
  booking advisory lock (retry-safe, distinct tips don't collapse).
- M-cap subtracts completed refunds, clamped to [0, total].

2FA (PSD2 SCA stand-in) for online saved-card payments:
- Full feature: status/setup/verify/disable endpoints, gating helper wired into
  all 7 saved-card charge paths (incl. BuyGiftCard + admin saved-card), account
  admin-tab settings UI, frontend gating across all payment surfaces.
- Enforcement is FAIL-CLOSED: on unless REQUIRE_2FA=false or an explicit
  mock/dev SQUARE_ENVIRONMENT; startup warning when off in a non-dev env.
- Verify is brute-force hardened (5-attempt lockout, timing-safe compare);
  plaintext codes only logged when enforcement is off (dev).
- GDPR: anonymize_user also scrubs 2FA columns and staff notes.

Infra/docs:
- nginx: /api/ response cache removed (cross-user disclosure); port 80
  redirects to HTTPS (localhost/RFC1918 exempt, end-anchored regexes); HSTS;
  separate webhook rate-limit zone.
- Schema: users 2FA columns; payments/till_sales square_source_id +
  square_request_snapshot.
- Legal docs: gift-card cooling-off, international-transfers section, tips
  policy; Gap Backlog P3 webhooks marked done; stale counts/wording corrected.
- Flaky test race fixed (t.Parallel + global mock mutation); suite 26/26
  packages green, 2,142 tests, svelte-check clean.

backend/handlers/user/account.go

@@ -21,6 +21,24 @@ import (
 	"github.com/jackc/pgx/v5"
 )
 
+// scrubAnonymizedUser2FA nulls the 2FA columns and staff notes that the SQL
+// anonymize_user() function does not scrub: it predates the 2FA columns and
+// intentionally preserves notes. A deleted user's live 2FA credential and any
+// PII in staff notes must not survive erasure, so run this inside the same
+// transaction as anonymize_user() to keep erasure atomic.
+func scrubAnonymizedUser2FA(ctx context.Context, q db.Querier, userID string) error {
+	_, err := q.Exec(ctx, `
+		UPDATE users
+		SET two_factor_enabled = FALSE,
+			two_factor_method = NULL,
+			two_factor_pending_code_hash = NULL,
+			two_factor_pending_code_expires = NULL,
+			notes = NULL
+		WHERE id = $1
+	`, userID)
+	return err
+}
+
 // DELETE /api/user/account
 func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 	userID, ok := mw.GetUserID(r.Context())
@@ -158,6 +176,15 @@ func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
+		// GDPR erasure gap: anonymize_user() leaves the 2FA columns and staff
+		// notes on the row. Scrub them here, in the same transaction, so erasure
+		// is atomic with the anonymization.
+		if err := scrubAnonymizedUser2FA(ctx, tx, userID); err != nil {
+			log.Printf("Failed to scrub 2FA fields for user %s: %v", userID, err)
+			http.Error(w, "server error", http.StatusInternalServerError)
+			return
+		}
+
 		if err := tx.Commit(ctx); err != nil {
 			log.Printf("Failed to commit transaction for user anonymization: %v", err)
 			http.Error(w, "server error", http.StatusInternalServerError)