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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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/servers.go

@@ -2,6 +2,8 @@ package storage
 
 import (
 	"fmt"
+	"os"
+	"path/filepath"
 	"time"
 
 	"github.com/qdm12/gluetun/internal/constants/providers"
@@ -10,12 +12,12 @@ import (
 
 // SetServers sets the given servers for the given provider
 // in the storage in-memory map and saves all the servers
-// to file.
+// to files.
 // Note the servers given are not copied so the caller must
 // NOT MUTATE them after calling this method.
 func (s *Storage) SetServers(provider string, servers []models.Server) (err error) {
 	if provider == providers.Custom {
-		return
+		return nil
 	}
 
 	s.mergedMutex.Lock()
@@ -26,10 +28,24 @@ func (s *Storage) SetServers(provider string, servers []models.Server) (err erro
 	serversObject.Servers = servers
 	s.mergedServers.ProviderToServers[provider] = serversObject
 
-	err = s.flushToFile(s.filepath)
+	if s.directoryPath == "" {
+		return nil // no disk writing
+	}
+
+	manifestPath := filepath.Join(s.directoryPath, manifestFilename)
+	err = s.flushToFile(manifestPath)
 	if err != nil {
 		return fmt.Errorf("saving servers to file: %w", err)
 	}
+
+	if !s.hasLegacy() {
+		return nil
+	}
+	s.logger.Infof("removing legacy %s which is now migrated to %s", s.legacyFilepath, s.directoryPath)
+	err = os.Remove(s.legacyFilepath)
+	if err != nil && !os.IsNotExist(err) {
+		s.logger.Warn("failed removing legacy servers file " + s.legacyFilepath + ": " + err.Error())
+	}
 	return nil
 }