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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,66 @@
+package protonvpn
+
+import (
+	"net/netip"
+
+	"github.com/qdm12/gluetun-servers/pkg/constants"
+	"github.com/qdm12/gluetun-servers/pkg/models"
+)
+
+type ipToServers map[string][2]models.Server // first server is OpenVPN, second is Wireguard.
+
+type features struct {
+	secureCore bool
+	tor        bool
+	p2p        bool
+	stream     bool
+}
+
+func (its ipToServers) add(country, region, city, name, hostname, wgPubKey string,
+	free bool, entryIP netip.Addr, features features,
+) {
+	key := entryIP.String()
+
+	servers, ok := its[key]
+	if ok {
+		return
+	}
+
+	baseServer := models.Server{
+		Country:     country,
+		Region:      region,
+		City:        city,
+		ServerName:  name,
+		Hostname:    hostname,
+		Free:        free,
+		SecureCore:  features.secureCore,
+		Tor:         features.tor,
+		PortForward: features.p2p,
+		Stream:      features.stream,
+		IPs:         []netip.Addr{entryIP},
+	}
+	openvpnServer := baseServer
+	openvpnServer.VPN = constants.OpenVPN
+	openvpnServer.UDP = true
+	openvpnServer.TCP = true
+	servers[0] = openvpnServer
+	wireguardServer := baseServer
+	wireguardServer.VPN = constants.Wireguard
+	wireguardServer.WgPubKey = wgPubKey
+	servers[1] = wireguardServer
+	its[key] = servers
+}
+
+func (its ipToServers) toServersSlice() (serversSlice []models.Server) {
+	const vpnProtocols = 2
+	serversSlice = make([]models.Server, 0, vpnProtocols*len(its))
+	for _, servers := range its {
+		serversSlice = append(serversSlice, servers[0], servers[1])
+	}
+	return serversSlice
+}
+
+func (its ipToServers) numberOfServers() int {
+	const serversPerIP = 2
+	return len(its) * serversPerIP
+}