backfill Subsonic to missing ServerTypes in config

backend/config.go

@@ -212,6 +212,15 @@ func ReadConfigFile(filepath, appVersionTag string) (*Config, error) {
 	if err := toml.NewDecoder(f).Decode(c); err != nil {
 		return nil, err
 	}
+
+	// Backfill Subsonic to empty ServerType fields
+	// for updating configs created before multiple MediaProviders were added
+	for _, s := range c.Servers {
+		if s.ServerType == "" {
+			s.ServerType = ServerTypeSubsonic
+		}
+	}
+
 	return c, nil
 }