feat(storage): storage file structure changes (#3301)

- migrate persisted server data storage from `/gluetun/servers.json` to `/gluetun/servers/`
- add `STORAGE_SERVERS_ENABLED=on` to enable or disable on-disk server data storage
- add `STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers` to configure where per-provider server files are stored
- keep backward compatibility with legacy `STORAGE_FILEPATH=/gluetun/servers.json`
- automatically read and migrate legacy `/gluetun/servers.json` into the new `/gluetun/servers/` layout when needed
- try to remove the legacy servers file after a successful migration to the new storage directory
- switch persisted server data from one large JSON file to a manifest plus per-provider JSON files
- add `UPDATER_PREFER_DIRECT_DOWNLOAD` to allow preferring direct download of provider server data
- keep deprecated updater flags `-enduser` and `-maintainer` as no-op warnings for backward compatibility
- preserve compatibility checks so persisted server data is discarded when its schema version no longer matches the built-in data
- allow preferred persisted provider data to override built-in data when versions match
- servers data now lives at https://github.com/qdm12/gluetun-servers/tree/main/pkg/servers

internal/storage/merge.go

@@ -30,6 +30,20 @@ func (s *Storage) mergeServers(hardcoded, persisted models.AllServers) models.Al
 func (s *Storage) mergeProviderServers(provider string,
 	hardcoded, persisted models.Servers,
 ) (merged models.Servers) {
+	if persisted.Preferred && persisted.Version != hardcoded.Version {
+		s.logger.Warn(fmt.Sprintf(
+			"persisted preferred %s servers are discarded because they have version %d and hardcoded servers have version %d",
+			provider, persisted.Version, hardcoded.Version))
+	}
+
+	// If persisted data is marked as preferred, use it regardless of timestamp
+	// (as long as versions match)
+	if persisted.Preferred && persisted.Version == hardcoded.Version && len(persisted.Servers) > 0 {
+		s.logger.Info(fmt.Sprintf(
+			"Using %s servers from file (marked as preferred)", provider))
+		return persisted
+	}
+
 	nowTimestamp := time.Now().Unix()
 	if persisted.Timestamp > nowTimestamp {
 		s.logger.Warn(fmt.Sprintf(