fix: use sql.NullString for reason field in audit log

Update reason field handling in auth.go and db.go to properly support
NULL values using sql.NullString. When inserting expired temp entries
and serializing logs to JSON, extract the string value to avoid
JSON marshaling issues with nullable fields.

db.go

@@ -205,11 +205,14 @@ func CleanupExpiredTemp(db *sql.DB) error {
 		return err
 	}
 
+	var reason string
+	reason = "expired"
+
 	for _, ip := range expiredIPs {
 		_, err = db.Exec(`
-			INSERT INTO audit_log(action, ip)
-			VALUES('expire_temp', ?)
-		`, ip)
+			INSERT INTO audit_log(action, ip, reason)
+			VALUES('expire_temp', ?, ?)
+		`, ip, reason)
 		if err != nil {
 			slog.Warn("failed to log expired temp entry", "ip", ip, "error", err)
 		}
@@ -231,15 +234,16 @@ func ListTempEntries(db *sql.DB) ([]map[string]interface{}, error) {
 	defer rows.Close()
 	var list []map[string]interface{} = make([]map[string]interface{}, 0)
 	for rows.Next() {
-		var ip, reason string
+		var ip string
 		var expiresAt time.Time
+		var reason sql.NullString
 		if err := rows.Scan(&ip, &expiresAt, &reason); err != nil {
 			continue
 		}
 		list = append(list, map[string]interface{}{
 			"ip":      ip,
 			"expires": expiresAt,
-			"reason":  reason,
+			"reason":  reason.String,
 		})
 	}
 	return list, nil