refactor: use slog.LevelVar for dynamic log levels

Replace the pattern of recreating the logger handler when enabling debug
logging. Now use slog.LevelVar which allows changing the log level
dynamically without recreating the handler or logger instance.

closes #8

internal/config/config.go

@@ -118,8 +118,6 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 
 	if s.Debug {
 		params.Logger.EnableDebugLogging()
-		s.log = params.Logger.Get()
-		log.Debug("Debug mode enabled")
 	}
 
 	// Log configuration summary (without secrets)