fix(security): remediate 2026-05-12 audit findings (Stages 2–5)

Remediation of the 2026-05-12/13 audits (78 findings + cluster gaps),
tracked in deploy-k3s/SECURITY.md, plus fixes from two independent
post-remediation reviews.

Auth & sessions:
- SHA-256 hashed auth-token storage (C1); prior-token cache eviction on
  re-login (MEDIUM-1)
- local Google JWKS verification, iss/aud/exp checks (C2/C3)
- constant-time login + generic errors (L1/LIVE-L11/LIVE-L13)
- per-account login lockout keyed on distinct source IPs (M5/MEDIUM-3)
- verified-email gating, login rate limiting (LIVE-L19, H1-H3)

IAP & webhooks:
- Apple/Google cross-account replay protection (C5/C6/C10/C13, H5/H6)
- migrations 000003-000006 (token hashing, IAP replay, audit_log +
  webhook_event_log table creation, append-only audit log)

Authorization & races:
- file-ownership owner-OR-member fix (C7), atomic share-code join
  (C9/H9), device-token reassignment (C8/LOW-3)

Secrets & deploy:
- secrets file-mounted at /etc/honeydue/secrets, not env (F8); Redis
  password out of the ConfigMap (HIGH-1); B2 keys reconciled
- digest-pinned images, admin ingress hardening, CSP/HSTS, /metrics
  lockdown; kubeconfig 0600, etcd secrets-encryption, fail2ban +
  unattended-upgrades at provision; secret-rotation runbook

Build, vet, and the full test suite (incl. -race) pass; the goose
migration chain is verified against PostgreSQL 16.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/services/contractor_service.go

@@ -296,9 +296,14 @@ func (s *ContractorService) ToggleFavorite(ctx context.Context, contractorID, us
 		return nil, apperrors.Internal(err)
 	}
 
-	// Re-fetch the contractor to get the updated state with all relations
+	// Re-fetch to get the updated state with all relations. Audit M12: if the
+	// contractor was deleted concurrently between the toggle and this read,
+	// surface a clean 404 instead of a 500.
 	contractor, err = s.contractorRepo.WithContext(ctx).FindByID(contractorID)
 	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, apperrors.NotFound("error.contractor_not_found")
+		}
 		return nil, apperrors.Internal(err)
 	}