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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,27 @@
+package html
+
+import (
+	"bytes"
+	"fmt"
+
+	"golang.org/x/net/html"
+)
+
+func WrapError(sentinelError error, node *html.Node) error {
+	return fmt.Errorf("%w: in HTML code: %s",
+		sentinelError, mustRenderHTML(node))
+}
+
+func WrapWarning(warning string, node *html.Node) string {
+	return fmt.Sprintf("%s: in HTML code: %s",
+		warning, mustRenderHTML(node))
+}
+
+func mustRenderHTML(node *html.Node) (rendered string) {
+	stringBuffer := bytes.NewBufferString("")
+	err := html.Render(stringBuffer, node)
+	if err != nil {
+		panic(err)
+	}
+	return stringBuffer.String()
+}