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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,19 @@
+package vyprvpn
+
+import (
+	"fmt"
+	"strings"
+)
+
+func parseFilename(fileName string) (
+	region string, err error,
+) {
+	const suffix = ".ovpn"
+	if !strings.HasSuffix(fileName, suffix) {
+		return "", fmt.Errorf("filename does not have the openvpn file extension: %s", fileName)
+	}
+
+	region = strings.TrimSuffix(fileName, suffix)
+	region = strings.ReplaceAll(region, " - ", " ")
+	return region, nil
+}