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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,28 @@
+package surfshark
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun-servers/pkg/updaters/providers/surfshark/servers"
+)
+
+func getHostInformation(host string, hostnameToLocation map[string]servers.ServerLocation) (
+	data servers.ServerLocation, err error,
+) {
+	locationData, ok := hostnameToLocation[host]
+	if !ok {
+		return locationData, fmt.Errorf("hostname %s not found in hostname to location mapping", host)
+	}
+
+	return locationData, nil
+}
+
+func hostToLocation(locationData []servers.ServerLocation) (
+	hostToLocation map[string]servers.ServerLocation,
+) {
+	hostToLocation = make(map[string]servers.ServerLocation, len(locationData))
+	for _, data := range locationData {
+		hostToLocation[data.Hostname] = data
+	}
+	return hostToLocation
+}