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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,21 @@
+package surfshark
+
+import (
+	"github.com/qdm12/gluetun-servers/pkg/updaters/providers/surfshark/servers"
+)
+
+// getRemainingServers finds extra servers not found in the API or in the ZIP file.
+func getRemainingServers(hts hostToServers) {
+	locationData := servers.LocationData()
+	hostnameToLocationLeft := hostToLocation(locationData)
+	for _, hostnameDone := range hts.toHostsSlice() {
+		delete(hostnameToLocationLeft, hostnameDone)
+	}
+
+	for hostname, locationData := range hostnameToLocationLeft {
+		// we assume the OpenVPN server supports both TCP and UDP
+		const tcp, udp = true, true
+		hts.addOpenVPN(hostname, locationData.Region, locationData.Country,
+			locationData.City, locationData.RetroLoc, tcp, udp)
+	}
+}