fix: add Debug() method to StructuredLog class

The Debug() method was missing from StructuredLog class, causing
compilation errors when trying to use debug-level structured logging.

This completes the log level API alongside existing Error(), Warn(),
Info(), and Critical() methods.

Author: libraz

src/utils/structured_log.h

@@ -169,6 +169,11 @@ class StructuredLog {
    */
   void Info() { spdlog::info("{}", Build()); }
 
+  /**
+   * @brief Log as debug level
+   */
+  void Debug() { spdlog::debug("{}", Build()); }
+
   /**
    * @brief Log as critical level
    */