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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,43 @@
+package html
+
+import (
+	"container/list"
+	"fmt"
+
+	"golang.org/x/net/html"
+)
+
+// BFS returns the node matching the match function and nil
+// if no node is found.
+func BFS(rootNode *html.Node, match MatchFunc) (node *html.Node) {
+	visited := make(map[*html.Node]struct{})
+	queue := list.New()
+	_ = queue.PushBack(rootNode)
+
+	for queue.Len() > 0 {
+		listElement := queue.Front()
+		node, ok := queue.Remove(listElement).(*html.Node)
+		if !ok {
+			panic(fmt.Sprintf("linked list has bad type %T", listElement.Value))
+		}
+
+		if node == nil {
+			continue
+		}
+
+		if _, ok := visited[node]; ok {
+			continue
+		}
+		visited[node] = struct{}{}
+
+		if match(node) {
+			return node
+		}
+
+		for child := node.FirstChild; child != nil; child = child.NextSibling {
+			_ = queue.PushBack(child)
+		}
+	}
+
+	return nil
+}