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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,56 @@
+package ipvanish
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+)
+
+func Test_parseFilename(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		fileName string
+		hostname string
+		country  string
+		city     string
+		err      error
+	}{
+		"unknown country code": {
+			fileName: "ipvanish-unknown-host.ovpn",
+			hostname: "host.ipvanish.com",
+			err:      errors.New("country code is unknown: unknown"),
+		},
+		"country code only": {
+			fileName: "ipvanish-ca-host.ovpn",
+			hostname: "host.ipvanish.com",
+			country:  "Canada",
+		},
+		"country code and city": {
+			fileName: "ipvanish-ca-sao-paulo-host.ovpn",
+			hostname: "host.ipvanish.com",
+			country:  "Canada",
+			city:     "Sao Paulo",
+		},
+	}
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			titleCaser := cases.Title(language.English)
+			country, city, err := parseFilename(testCase.fileName, testCase.hostname, titleCaser)
+
+			if testCase.err != nil {
+				require.Error(t, err)
+				assert.Equal(t, testCase.err.Error(), err.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.Equal(t, testCase.country, country)
+			assert.Equal(t, testCase.city, city)
+		})
+	}
+}