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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,64 @@
+package airvpn
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/netip"
+)
+
+type apiData struct {
+	Servers []apiServer `json:"servers"`
+}
+
+type apiServer struct {
+	PublicName  string     `json:"public_name"`
+	CountryName string     `json:"country_name"`
+	CountryCode string     `json:"country_code"`
+	Location    string     `json:"location"`
+	Continent   string     `json:"continent"`
+	IPv4In1     netip.Addr `json:"ip_v4_in1"`
+	IPv4In2     netip.Addr `json:"ip_v4_in2"`
+	IPv4In3     netip.Addr `json:"ip_v4_in3"`
+	IPv4In4     netip.Addr `json:"ip_v4_in4"`
+	IPv6In1     netip.Addr `json:"ip_v6_in1"`
+	IPv6In2     netip.Addr `json:"ip_v6_in2"`
+	IPv6In3     netip.Addr `json:"ip_v6_in3"`
+	IPv6In4     netip.Addr `json:"ip_v6_in4"`
+	Health      string     `json:"health"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error,
+) {
+	const url = "https://airvpn.org/api/status/"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, fmt.Errorf("creating HTTP request: %w", err)
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, fmt.Errorf("doing HTTP request: %w", err)
+	}
+
+	if response.StatusCode != http.StatusOK {
+		_ = response.Body.Close()
+		return data, 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 {
+		_ = response.Body.Close()
+		return data, fmt.Errorf("decoding response body: %w", err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return data, fmt.Errorf("closing response body: %w", err)
+	}
+
+	return data, nil
+}