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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -132,7 +132,7 @@ func (s *Settings) SetDefaults() {
 	s.IPv6.setDefaults()
 	s.PublicIP.setDefaults()
 	s.Shadowsocks.setDefaults()
-	s.Storage.setDefaults()
+	s.Storage.SetDefaults()
 	s.System.setDefaults()
 	s.Version.setDefaults()
 	s.VPN.setDefaults()
@@ -213,7 +213,7 @@ func (s *Settings) Read(r *reader.Reader, warner Warner) (err error) {
 			return s.PublicIP.read(r, warner)
 		},
 		"shadowsocks": s.Shadowsocks.read,
-		"storage":     s.Storage.read,
+		"storage":     s.Storage.Read,
 		"system":      s.System.read,
 		"updater":     s.Updater.read,
 		"version":     s.Version.read,