Add multi-server data model + legacy DB migration (step 0)

Adds Server.ID/Name, ServerInterface.Name, Client.ServerID, and a new
ServerSetting type as additive fields so existing single-server code
paths keep working unchanged. jsondb.Init() now detects a pre-existing
single-server db/server/ layout and mirrors it into new servers/,
server_settings/, server_hashes/ collections plus backfills ServerID
on existing clients, without touching/removing the legacy files yet.

model/client.go

@@ -7,6 +7,7 @@ import (
 // Client model
 type Client struct {
 	ID              string    `json:"id"`
+	ServerID        string    `json:"server_id,omitempty"`
 	PrivateKey      string    `json:"private_key"`
 	PublicKey       string    `json:"public_key"`
 	PresharedKey    string    `json:"preshared_key"`

model/server.go

@@ -6,6 +6,8 @@ import (
 
 // Server model
 type Server struct {
+	ID        string `json:"id,omitempty"`
+	Name      string `json:"name,omitempty"`
 	KeyPair   *ServerKeypair
 	Interface *ServerInterface
 }
@@ -19,6 +21,7 @@ type ServerKeypair struct {
 
 // ServerInterface model
 type ServerInterface struct {
+	Name       string    `json:"name,omitempty"` // OS interface name, e.g. wg0
 	Addresses  []string  `json:"addresses"`
 	ListenPort int       `json:"listen_port,string"` // ,string to get listen_port string input as int
 	UpdatedAt  time.Time `json:"updated_at"`

model/setting.go

@@ -15,3 +15,14 @@ type GlobalSetting struct {
 	ConfigFilePath      string    `json:"config_file_path"`
 	UpdatedAt           time.Time `json:"updated_at"`
 }
+
+// ServerSetting model holds the per-server-interface subset of settings
+// that used to live in the single global GlobalSetting. Introduced for the
+// multi-server extension; not yet wired into store/handler layers.
+type ServerSetting struct {
+	EndpointAddress string    `json:"endpoint_address"`
+	FirewallMark    string    `json:"firewall_mark"`
+	Table           string    `json:"table"`
+	ConfigFilePath  string    `json:"config_file_path"`
+	UpdatedAt       time.Time `json:"updated_at"`
+}