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/residence_repo.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/rs/zerolog/log"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 
 	"github.com/treytartt/honeydue-api/internal/models"
 )
@@ -194,6 +195,60 @@ func (r *ResidenceRepository) HasAccess(residenceID, userID uint) (bool, error)
 	return count > 0, nil
 }
 
+// JoinWithShareCode atomically redeems a one-time share code (audit C9/H9):
+// it locks the share-code row, re-checks validity under the lock, adds the
+// user to the residence, and deactivates the code — all in one transaction.
+// Concurrent redemptions of the same code serialize on the row lock; the
+// loser sees is_active=false and is rejected. A failure to deactivate aborts
+// the whole join. Returns gorm.ErrRecordNotFound for an unknown, inactive, or
+// expired code so the caller can map every case to one generic error.
+func (r *ResidenceRepository) JoinWithShareCode(code string, userID uint) (residenceID uint, alreadyMember bool, err error) {
+	err = r.db.Transaction(func(tx *gorm.DB) error {
+		var sc models.ResidenceShareCode
+		if e := tx.Clauses(clause.Locking{Strength: "UPDATE"}).
+			Where("code = ?", code).First(&sc).Error; e != nil {
+			return e
+		}
+		if !sc.IsActive || (sc.ExpiresAt != nil && time.Now().UTC().After(*sc.ExpiresAt)) {
+			return gorm.ErrRecordNotFound
+		}
+		residenceID = sc.ResidenceID
+
+		// Already a member (owner or shared user)?
+		var accessCount int64
+		if e := tx.Raw(`
+			SELECT COUNT(*) FROM (
+				SELECT 1 FROM residence_residence
+				WHERE id = ? AND owner_id = ? AND is_active = true
+				UNION
+				SELECT 1 FROM residence_residence_users
+				WHERE residence_id = ? AND user_id = ?
+			) ac
+		`, sc.ResidenceID, userID, sc.ResidenceID, userID).Scan(&accessCount).Error; e != nil {
+			return e
+		}
+		if accessCount > 0 {
+			alreadyMember = true
+			return nil
+		}
+
+		if e := tx.Exec(
+			"INSERT INTO residence_residence_users (residence_id, user_id) VALUES (?, ?) ON CONFLICT DO NOTHING",
+			sc.ResidenceID, userID,
+		).Error; e != nil {
+			return e
+		}
+
+		// One-time use: deactivate the code. A failure here aborts the join.
+		if e := tx.Model(&models.ResidenceShareCode{}).
+			Where("id = ?", sc.ID).Update("is_active", false).Error; e != nil {
+			return e
+		}
+		return nil
+	})
+	return residenceID, alreadyMember, err
+}
+
 // IsOwner checks if a user is the owner of a residence
 func (r *ResidenceRepository) IsOwner(residenceID, userID uint) (bool, error) {
 	var count int64