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/repositories/subscription_repo.go

@@ -151,6 +151,28 @@ func (r *SubscriptionRepository) FindByAppleReceiptContains(transactionID string
 	return &sub, nil
 }
 
+// FindByAppleOriginalTransactionID finds a subscription by the Apple original
+// transaction ID (audit C5/C13). Exact match on an indexed column — replaces
+// the LIKE scan in FindByAppleReceiptContains for both replay detection and
+// webhook user lookup.
+func (r *SubscriptionRepository) FindByAppleOriginalTransactionID(originalTransactionID string) (*models.UserSubscription, error) {
+	var sub models.UserSubscription
+	err := r.db.Where("apple_original_transaction_id = ?", originalTransactionID).First(&sub).Error
+	if err != nil {
+		return nil, err
+	}
+	return &sub, nil
+}
+
+// UpdateAppleOriginalTransactionID binds an Apple original transaction ID to a
+// user's subscription. A partial unique index enforces one account per
+// transaction (audit C5) — a second account claiming the same ID fails here.
+func (r *SubscriptionRepository) UpdateAppleOriginalTransactionID(userID uint, originalTransactionID string) error {
+	return r.db.Model(&models.UserSubscription{}).
+		Where("user_id = ?", userID).
+		Update("apple_original_transaction_id", originalTransactionID).Error
+}
+
 // FindByGoogleToken finds a subscription by Google purchase token
 // Used by webhooks to find the user associated with a purchase
 func (r *SubscriptionRepository) FindByGoogleToken(purchaseToken string) (*models.UserSubscription, error) {