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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -155,6 +155,8 @@ func (a *AllServers) Count() (count int) {
 type Servers struct {
 	Version   uint16   `json:"version"`
 	Timestamp int64    `json:"timestamp"`
+	Preferred bool     `json:"preferred,omitempty"`
+	Filepath  string   `json:"filepath,omitempty"`
 	Servers   []Server `json:"servers,omitempty"`
 }