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.

store/jsondb/jsondb.go

@@ -4,9 +4,11 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"log"
 	"os"
 	"path"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/sdomino/scribble"
@@ -17,6 +19,10 @@ import (
 	"github.com/ngoduykhanh/wireguard-ui/util"
 )
 
+// legacyDefaultServerID is the synthetic ID assigned to a pre-existing
+// single-server installation when it is migrated to the multi-server layout.
+const legacyDefaultServerID = "wg0"
+
 type JsonDB struct {
 	conn   *scribble.Driver
 	dbPath string
@@ -161,6 +167,11 @@ func (o *JsonDB) Init() error {
 		}
 	}
 
+	// migrate legacy single-server layout to the new multi-server layout
+	if err := o.migrateLegacyServer(); err != nil {
+		return err
+	}
+
 	// init cache
 	for _, i := range results {
 		user := model.User{}
@@ -186,6 +197,122 @@ func (o *JsonDB) Init() error {
 	return nil
 }
 
+// migrateLegacyServer detects a pre-multi-server DB layout (single
+// "server" collection, no "servers" registry yet) and migrates it into
+// the new per-server layout ("servers/<id>.json", "server_settings/<id>.json",
+// "server_hashes/<id>.json"), backfilling ServerID on every existing client.
+// It is idempotent: it only runs once, guarded by the absence of the
+// "servers" directory, and never deletes or modifies the legacy "server"
+// collection - existing store methods keep reading/writing it directly
+// until they are switched over to the new layout in a later step.
+func (o *JsonDB) migrateLegacyServer() error {
+	legacyServerPath := path.Join(o.dbPath, "server")
+	legacyInterfacePath := path.Join(legacyServerPath, "interfaces.json")
+	newServersPath := path.Join(o.dbPath, "servers")
+
+	if _, err := os.Stat(legacyInterfacePath); os.IsNotExist(err) {
+		// nothing to migrate (fresh install)
+		return nil
+	}
+	if _, err := os.Stat(newServersPath); err == nil {
+		// already migrated
+		return nil
+	}
+
+	var legacyInterface model.ServerInterface
+	if err := o.conn.Read("server", "interfaces", &legacyInterface); err != nil {
+		return fmt.Errorf("migration: cannot read legacy server interface: %v", err)
+	}
+	var legacyKeyPair model.ServerKeypair
+	if err := o.conn.Read("server", "keypair", &legacyKeyPair); err != nil {
+		return fmt.Errorf("migration: cannot read legacy server keypair: %v", err)
+	}
+	var legacyGlobalSettings model.GlobalSetting
+	if err := o.conn.Read("server", "global_settings", &legacyGlobalSettings); err != nil {
+		return fmt.Errorf("migration: cannot read legacy global settings: %v", err)
+	}
+	var legacyHashes model.ClientServerHashes
+	hasHashes := true
+	if err := o.conn.Read("server", "hashes", &legacyHashes); err != nil {
+		hasHashes = false
+	}
+
+	serverID := legacyDefaultServerID
+	ifaceName := legacyDefaultServerID
+	if legacyGlobalSettings.ConfigFilePath != "" {
+		base := path.Base(legacyGlobalSettings.ConfigFilePath)
+		if strings.HasSuffix(base, ".conf") {
+			ifaceName = strings.TrimSuffix(base, ".conf")
+		}
+	}
+
+	legacyInterface.Name = ifaceName
+
+	server := model.Server{
+		ID:        serverID,
+		Name:      "Default Server",
+		KeyPair:   &legacyKeyPair,
+		Interface: &legacyInterface,
+	}
+	if err := o.conn.Write("servers", serverID, server); err != nil {
+		return fmt.Errorf("migration: cannot write servers/%s.json: %v", serverID, err)
+	}
+	if err := util.ManagePerms(path.Join(newServersPath, serverID+".json")); err != nil {
+		return err
+	}
+
+	serverSetting := model.ServerSetting{
+		EndpointAddress: legacyGlobalSettings.EndpointAddress,
+		FirewallMark:    legacyGlobalSettings.FirewallMark,
+		Table:           legacyGlobalSettings.Table,
+		ConfigFilePath:  legacyGlobalSettings.ConfigFilePath,
+		UpdatedAt:       time.Now().UTC(),
+	}
+	if err := o.conn.Write("server_settings", serverID, serverSetting); err != nil {
+		return fmt.Errorf("migration: cannot write server_settings/%s.json: %v", serverID, err)
+	}
+	if err := util.ManagePerms(path.Join(o.dbPath, "server_settings", serverID+".json")); err != nil {
+		return err
+	}
+
+	if hasHashes {
+		if err := o.conn.Write("server_hashes", serverID, legacyHashes); err != nil {
+			return fmt.Errorf("migration: cannot write server_hashes/%s.json: %v", serverID, err)
+		}
+		if err := util.ManagePerms(path.Join(o.dbPath, "server_hashes", serverID+".json")); err != nil {
+			return err
+		}
+	}
+
+	// backfill ServerID on every existing client
+	clientRecords, err := o.conn.ReadAll("clients")
+	if err != nil && err != scribble.ErrMissingCollection {
+		return fmt.Errorf("migration: cannot read clients: %v", err)
+	}
+	for _, rec := range clientRecords {
+		var client model.Client
+		if err := json.Unmarshal(rec, &client); err != nil {
+			return fmt.Errorf("migration: cannot decode client json: %v", err)
+		}
+		if client.ServerID != "" {
+			continue
+		}
+		client.ServerID = serverID
+		if err := o.conn.Write("clients", client.ID, client); err != nil {
+			return fmt.Errorf("migration: cannot backfill server_id on client %s: %v", client.ID, err)
+		}
+	}
+
+	// NOTE: the legacy "server" directory is intentionally left in place
+	// (not renamed/removed) at this stage. Existing store methods (GetServer,
+	// GetGlobalSettings, SaveServerInterface, ...) still read/write it
+	// directly until they are switched over to the new per-server layout;
+	// removing it now would break the running app. The rename-to-backup
+	// step happens once those methods are migrated.
+	log.Printf("Migrated legacy single-server DB to multi-server layout (server id: %s); legacy db/server/ kept in place for now", serverID)
+	return nil
+}
+
 // GetUsers func to get all users from the database
 func (o *JsonDB) GetUsers() ([]model.User, error) {
 	var users []model.User