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/user_repo.go

@@ -174,10 +174,12 @@ func (r *UserRepository) GetOrCreateToken(userID uint) (*models.AuthToken, error
 	return &token, nil
 }
 
-// FindTokenByKey looks up an auth token by its key value.
-func (r *UserRepository) FindTokenByKey(key string) (*models.AuthToken, error) {
+// FindTokenByKey looks up an auth token by its raw key value. The raw token
+// is hashed (audit C1) before the indexed lookup, since only the hash is
+// stored.
+func (r *UserRepository) FindTokenByKey(rawKey string) (*models.AuthToken, error) {
 	var token models.AuthToken
-	if err := r.db.Where("key = ?", key).First(&token).Error; err != nil {
+	if err := r.db.Where("key = ?", models.HashToken(rawKey)).First(&token).Error; err != nil {
 		if errors.Is(err, gorm.ErrRecordNotFound) {
 			return nil, ErrTokenNotFound
 		}
@@ -195,9 +197,46 @@ func (r *UserRepository) CreateToken(userID uint) (*models.AuthToken, error) {
 	return &token, nil
 }
 
-// DeleteToken deletes an auth token
+// CreateFreshToken issues a new auth token for the user, replacing any
+// existing one. Because tokens are stored hashed (audit C1) the server
+// cannot re-issue a previously-minted token's plaintext, so every login
+// mints a fresh token. The returned token's Plaintext field carries the
+// raw value to hand to the client; it is never persisted.
+//
+// It also returns the stored hashes of the token rows it deleted, so the
+// caller can evict those entries from the Redis token cache (audit MEDIUM-1).
+// Without that, a prior (e.g. stolen) token keeps authenticating via a cache
+// hit for up to the cache TTL even though its DB row is gone.
+func (r *UserRepository) CreateFreshToken(userID uint) (*models.AuthToken, []string, error) {
+	var token models.AuthToken
+	var oldHashes []string
+	err := r.db.Transaction(func(tx *gorm.DB) error {
+		var old []models.AuthToken
+		if err := tx.Where("user_id = ?", userID).Find(&old).Error; err != nil {
+			return err
+		}
+		oldHashes = make([]string, 0, len(old))
+		for i := range old {
+			if old[i].Key != "" {
+				oldHashes = append(oldHashes, old[i].Key)
+			}
+		}
+		if err := tx.Where("user_id = ?", userID).Delete(&models.AuthToken{}).Error; err != nil {
+			return err
+		}
+		token = models.AuthToken{UserID: userID}
+		return tx.Create(&token).Error
+	})
+	if err != nil {
+		return nil, nil, err
+	}
+	return &token, oldHashes, nil
+}
+
+// DeleteToken deletes an auth token by its raw key value. The raw token is
+// hashed (audit C1) before the lookup, since only the hash is stored.
 func (r *UserRepository) DeleteToken(token string) error {
-	result := r.db.Where("key = ?", token).Delete(&models.AuthToken{})
+	result := r.db.Where("key = ?", models.HashToken(token)).Delete(&models.AuthToken{})
 	if result.Error != nil {
 		return result.Error
 	}