fix: fresh-review round — 2FA deliverability, disable re-verification, GDPR batch scrub, dispute alerting, docs accuracy

Second fresh-eyes review pass (7 agents: goal, security, code-quality,
context-mining, webhooks+2FA, client+mock+sweep, refunds/giftcards/handlers).
Money-safety core verified sound (identical-body replay byte-lossless, clawback
gated on definitive proof, no double-charge window). This round fixes the
issues the fresh pass surfaced:

2FA:
- Setup now DELIVERS the code via the [2FA] server log in ALL modes (was:
  nothing in enforced mode -> production 2FA was an unbreakable dead-end and
  saved-card charges were permanently 403). Enforced mode still withholds the
  code from the API response; the log line is the fake delivery channel until
  email/SMS lands (P6).
- Disabling 2FA now requires a fresh verification code when enforcement is ON
  (previously ignored the code -> a password-only attacker could lift the gate).
  Shares the 5-attempt lockout and timing-safe compare. Dev bypass retained.
- REQUIRE_2FA parsing normalized (false/0/off/no, case-insensitive);
  startup warning extended to the empty-env/mock-client/enforced-2FA confusion.

GDPR:
- anonymize_user() SQL now scrubs two_factor_* columns + staff notes, so the
  idle-account batch cleanup (CleanupIdleAccounts) is erasure-clean, not just
  the user-initiated delete path.

Webhooks:
- dispute.created for an untracked Square payment now raises a
  critical_payment_log admin notification (chargeback the app can't reconcile
  is never silent). Reason strings truncated on rune boundaries (valid UTF-8).
  Stale at-most-once comment corrected; revertTillSaleGiftCardFunding
  duplication noted.

Sweep/mock parity:
- Mock CreatePayment dedup is now source-aware (IDEMPOTENCY_KEY_REUSED on
  source mismatch) matching ReplayPaymentByKey and real Square.
- COMPLETED-but-never-polled terminal till-sale checkouts are now recorded by
  the sweep (previously only booking checkouts were; till charges were
  invisible until the 24h blind-fail WARN).
- Legacy snapshot-less minimal-body replay, SQUARE_LOCATION_ID drift, and
  in-memory-mock-restart limitations documented.

Docs:
- Webhook path corrected everywhere (/webhooks/square, not /api/webhooks/square
  - a deployer following the old path would 404 and silently lose all webhook
  reconciliation).
- 2FA enforcement semantics + code-delivery mechanism documented accurately
  (fail-closed default; log-delivery channel; disable re-verification).
- README/User Manual note the 2FA requirement on online saved-card payments.

Tests: 2,151 (up from 2,142). Backend 26/27 packages green (crussell/db fails
only in this environment: local postgres doesn't offer scram-sha-256 for the
test role; package is byte-identical to HEAD and untouched here). Frontend
builds; svelte-check 0 errors.

README.md

@@ -68,6 +68,10 @@ docker compose up --build -d
 
 `SQUARE_WEBHOOK_NOTIFICATION_URL` and `SQUARE_WEBHOOK_SIGNATURE_KEY` in `.env` must exactly match the webhook subscription configured in the Square Dashboard. An unset URL defaults to `http://localhost:8080/webhooks/square`, which is fail-closed (503 without the signing key, 403 on missing/bad signature). If you don't need webhooks, leave both empty — the handler still rejects cleanly.
 
+### Two-factor authentication (2FA)
+
+`REQUIRE_2FA` gates saved-card online payments (PSD2 SCA stand-in) and is **fail-closed**: enforcement is ON by default for any `SQUARE_ENVIRONMENT` except an explicit `mock`/`dev`/`development`/`test` value — empty or unknown values are treated as production-enforced. Disable it with `REQUIRE_2FA=false` or an explicit mock env. The 6-digit code is delivered via the server log (`[2FA]` prefix; the operator relays it) until email/SMS lands.
+
 ### Local dev (tmux)
 
 ```bash
@@ -85,7 +89,7 @@ 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 ./...   # 2,133 tests passed (4 skipped, ~2min)
+cd backend && go test -tags "test,dev" -count=1 -parallel 8 ./...   # 2,142 tests passed (4 skipped, ~2min)
 cd backend && go test -tags "test,dev" -count=1 -race -timeout 480s ./...  # race detector (all packages, ~4min)
 cd backend && go test -tags "test,dev" -count=10 -parallel 8 ./...  # thorough verification (~2-3min)
 ```