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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,38 @@
+package models
+
+type SortableServers []Server
+
+func (s SortableServers) Len() int {
+	return len(s)
+}
+
+func (s SortableServers) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+func (s SortableServers) Less(i, j int) bool {
+	a, b := s[i], s[j]
+
+	if a.Country == b.Country { //nolint:nestif
+		if a.Region == b.Region {
+			if a.City == b.City {
+				if a.ServerName == b.ServerName {
+					if a.Number == b.Number {
+						if a.Hostname == b.Hostname {
+							if a.ISP == b.ISP {
+								return a.VPN < b.VPN
+							}
+							return a.ISP < b.ISP
+						}
+						return a.Hostname < b.Hostname
+					}
+					return a.Number < b.Number
+				}
+				return a.ServerName < b.ServerName
+			}
+			return a.City < b.City
+		}
+		return a.Region < b.Region
+	}
+	return a.Country < b.Country
+}