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.

auth.go

@@ -182,8 +182,8 @@ func logsHandler(cfg Config, db *sql.DB) http.HandlerFunc {
 		logs := make([]map[string]interface{}, 0)
 		for rows.Next() {
 			var ts time.Time
-			var action, apiClientIP, reason string
-			var ip, cidr sql.NullString
+			var action, apiClientIP string
+			var ip, cidr, reason sql.NullString
 			var ttlSeconds sql.NullInt64
 			if err := rows.Scan(&ts, &action, &ip, &cidr, &ttlSeconds, &reason, &apiClientIP); err != nil {
 				continue
@@ -192,7 +192,7 @@ func logsHandler(cfg Config, db *sql.DB) http.HandlerFunc {
 				"timestamp":     ts,
 				"action":        action,
 				"api_client_ip": apiClientIP,
-				"reason":        reason,
+				"reason":        reason.String,
 			}
 			if ip.Valid {
 				entry["ip"] = ip.String