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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,32 @@
+package models
+
+import (
+	"net/netip"
+)
+
+type PublicIP struct {
+	IP           netip.Addr `json:"public_ip"`
+	Region       string     `json:"region,omitempty"`
+	Country      string     `json:"country,omitempty"`
+	City         string     `json:"city,omitempty"`
+	Hostname     string     `json:"hostname,omitempty"`
+	Location     string     `json:"location,omitempty"`
+	Organization string     `json:"organization,omitempty"`
+	PostalCode   string     `json:"postal_code,omitempty"`
+	Timezone     string     `json:"timezone,omitempty"`
+}
+
+func (p *PublicIP) Copy() (publicIPCopy PublicIP) {
+	publicIPCopy = PublicIP{
+		IP:           p.IP,
+		Region:       p.Region,
+		Country:      p.Country,
+		City:         p.City,
+		Hostname:     p.Hostname,
+		Location:     p.Location,
+		Organization: p.Organization,
+		PostalCode:   p.PostalCode,
+		Timezone:     p.Timezone,
+	}
+	return publicIPCopy
+}