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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,58 @@
+package vpnsecure
+
+import (
+	"context"
+	"fmt"
+	"sort"
+
+	"github.com/qdm12/gluetun-servers/pkg/constants"
+	"github.com/qdm12/gluetun-servers/pkg/models"
+	"github.com/qdm12/gluetun-servers/pkg/updaters/common"
+)
+
+func (u *Updater) FetchServers(ctx context.Context, minServers int) (
+	servers []models.Server, err error,
+) {
+	servers, err = fetchServers(ctx, u.client, u.warner)
+	if err != nil {
+		return nil, fmt.Errorf("fetching servers: %w", err)
+	} else if len(servers) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	hts := make(hostToServer, len(servers))
+	for _, server := range servers {
+		hts[server.Hostname] = server
+	}
+
+	hosts := hts.toHostsSlice()
+
+	resolveSettings := parallelResolverSettings(hosts)
+	hostToIPs, warnings, err := u.parallelResolver.Resolve(ctx, resolveSettings)
+	for _, warning := range warnings {
+		u.warner.Warn(warning)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if len(hostToIPs) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	hts.adaptWithIPs(hostToIPs)
+
+	servers = hts.toServersSlice()
+
+	for i := range servers {
+		servers[i].VPN = constants.OpenVPN
+		servers[i].UDP = true
+		servers[i].TCP = true
+	}
+
+	sort.Sort(models.SortableServers(servers))
+
+	return servers, nil
+}