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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,72 @@
+package privateinternetaccess
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/netip"
+)
+
+type apiData struct {
+	Regions []regionData `json:"regions"`
+}
+
+type regionData struct {
+	Name        string `json:"name"`
+	DNS         string `json:"dns"`
+	PortForward bool   `json:"port_forward"`
+	Offline     bool   `json:"offline"`
+	Servers     struct {
+		UDP []serverData `json:"ovpnudp"`
+		TCP []serverData `json:"ovpntcp"`
+	} `json:"servers"`
+}
+
+type serverData struct {
+	IP netip.Addr `json:"ip"`
+	CN string     `json:"cn"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error,
+) {
+	const url = "https://serverlist.piaservers.net/vpninfo/servers/v7"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return data, fmt.Errorf("HTTP status code not OK: %d %s",
+			response.StatusCode, response.Status)
+	}
+
+	b, err := io.ReadAll(response.Body)
+	if err != nil {
+		return data, err
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return data, err
+	}
+
+	// remove key/signature at the bottom
+	i := bytes.IndexRune(b, '\n')
+	b = b[:i]
+
+	if err := json.Unmarshal(b, &data); err != nil {
+		return data, err
+	}
+
+	return data, nil
+}