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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,40 @@
+package resolver
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_uniqueIPsToSlice(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		inputIPs  map[string]struct{}
+		outputIPs []netip.Addr
+	}{
+		"nil": {
+			inputIPs:  nil,
+			outputIPs: []netip.Addr{},
+		},
+		"empty": {
+			inputIPs:  map[string]struct{}{},
+			outputIPs: []netip.Addr{},
+		},
+		"single IPv4": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}},
+			outputIPs: []netip.Addr{netip.AddrFrom4([4]byte{1, 1, 1, 1})},
+		},
+		"two IPv4s": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}, "1.1.2.1": {}},
+			outputIPs: []netip.Addr{netip.AddrFrom4([4]byte{1, 1, 1, 1}), netip.AddrFrom4([4]byte{1, 1, 2, 1})},
+		},
+	}
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			outputIPs := uniqueIPsToSlice(testCase.inputIPs)
+			assert.ElementsMatch(t, testCase.outputIPs, outputIPs)
+		})
+	}
+}