Add optional "log_file" configuration option.

config.go

@@ -17,6 +17,9 @@ type Config struct {
 	// DataDir is the base directory for all database files.
 	DataDir string `json:"data_dir"`
 
+	// LogFile is the optional path to a file where logs will be written.
+	LogFile string `json:"log_file"`
+
 	// HotRetentionHours is how many hours of raw ticks to keep in the hot DB.
 	HotRetentionHours int `json:"hot_retention_hours"`
 
@@ -42,6 +45,7 @@ func DefaultConfig() Config {
 		WebSocketURL:               "wss://stream.bybit.com/v5/public/linear",
 		Symbol:                     "BTCUSDT",
 		DataDir:                    "data",
+		LogFile:                    "engine.log",
 		HotRetentionHours:          12,
 		FeatureRetentionDays:       30,
 		WriterFlushIntervalMs:      500,
@@ -52,14 +56,12 @@ func DefaultConfig() Config {
 }
 
 // LoadConfig reads a JSON config file and merges with defaults.
-// If the file does not exist, defaults are returned and a config file is written.
 func LoadConfig(path string) (Config, error) {
 	cfg := DefaultConfig()
 
 	data, err := os.ReadFile(path)
 	if err != nil {
 		if os.IsNotExist(err) {
-			// Write default config for user reference
 			if writeErr := writeDefaultConfig(path, cfg); writeErr != nil {
 				return cfg, fmt.Errorf("failed to write default config: %w", writeErr)
 			}