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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,51 @@
+package mullvad
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+type serverData struct {
+	Hostname string `json:"hostname"`
+	Country  string `json:"country_name"`
+	City     string `json:"city_name"`
+	Active   bool   `json:"active"`
+	Owned    bool   `json:"owned"`
+	Provider string `json:"provider"`
+	IPv4     string `json:"ipv4_addr_in"`
+	IPv6     string `json:"ipv6_addr_in"`
+	Type     string `json:"type"`
+	PubKey   string `json:"pubkey"` // Wireguard public key
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (data []serverData, err error) {
+	const url = "https://api.mullvad.net/www/relays/all/"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("HTTP status code not OK: %d %s", response.StatusCode, response.Status)
+	}
+
+	decoder := json.NewDecoder(response.Body)
+	if err := decoder.Decode(&data); err != nil {
+		return nil, fmt.Errorf("failed decoding response body: %s", err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	return data, nil
+}