add: SQLite-backed structured logging with retention

Replace JSON/file logging with a logs.db (WAL) store. Thread the logger
through the encoder and metadata client for debug instrumentation of every
ffmpeg/ffprobe/opusenc invocation and OMDb/TVmaze request. API keys are
redacted before request URLs are logged. Retention defaults to 7 days,
overridable via log_retention_days in config.

internal/config/config.go

@@ -76,6 +76,9 @@ func Load(configPath string) (*types.Config, error) {
 	if cfg.Encoding.TVRip.Preset == 0 {
 		cfg.Encoding.TVRip.Preset = 2
 	}
+	if cfg.LogRetentionDays == 0 {
+		cfg.LogRetentionDays = 7
+	}
 
 	return &cfg, nil
 }