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/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"`
+}