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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,26 @@
+package vpnsecure
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/net/html"
+)
+
+func parseTestHTML(t *testing.T, htmlString string) *html.Node {
+	t.Helper()
+	rootNode, err := html.Parse(strings.NewReader(htmlString))
+	require.NoError(t, err)
+	return rootNode
+}
+
+func parseTestDataIndexHTML(t *testing.T) *html.Node {
+	t.Helper()
+
+	data, err := os.ReadFile("testdata/index.html")
+	require.NoError(t, err)
+
+	return parseTestHTML(t, string(data))
+}