refactor(storage): new storage file structure

- new directory structure containing manifest.json and one json file per provider, by default.
- the manifest.json file can specify a filepath for each vpn provider
- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch
- `STORAGE_SERVERS_DIRECTORY_PATH` replaces `STORAGE_FILEPATH` (which is now a migration source only). It sets the directory where server manifest and per-provider JSON files are stored (default: `/gluetun/servers/`).
- First-run migration: On startup, gluetun checks for the old /gluetun/servers.json file; if found and no new manifest exists, it automatically migrates all data to /gluetun/servers/ directory structure
- Silent fallback: If legacy file isn't found, uses the new directory path normally
- Legacy cleanup: After successful migration, attempts to remove the old fat JSON file (logs warning only if removal fails, e.g., read-only bind mounts)

Co-authored-by: Copilot <copilot@github.com>

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(