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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

gluetun-servers/pkg/updaters/providers/nordvpn/name.go

@@ -0,0 +1,26 @@
+package nordvpn
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+var ErrNoIDInServerName = errors.New("no ID in server name")
+
+func parseServerName(serverName string) (number uint16, err error) {
+	i := strings.IndexRune(serverName, '#')
+	if i < 0 {
+		return 0, fmt.Errorf("%w: %s", ErrNoIDInServerName, serverName)
+	}
+
+	idString := serverName[i+1:]
+	idUint64, err := strconv.ParseUint(idString, 10, 16)
+	if err != nil {
+		return 0, fmt.Errorf("invalid ID in server name: %s", serverName)
+	}
+
+	number = uint16(idUint64)
+	return number, nil
+}