fix: 发布产物自包含数据库迁移资源，修复 macOS DMG 安装后无法启动

使用 go:embed 嵌入迁移 SQL 到二进制，移除 runtime.Caller 源码路径依赖，
server 和 desktop 发布产物均可在无源码目录环境下完成数据库初始化和迁移。

backend/internal/database/database.go

@@ -1,10 +1,10 @@
 package database
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 
 	"github.com/pressly/goose/v3"
 	"go.uber.org/zap"
@@ -13,6 +13,7 @@ import (
 	"gorm.io/gorm"
 
 	"nex/backend/internal/config"
+	"nex/backend/migrations"
 	pkglogger "nex/backend/pkg/logger"
 )
 
@@ -77,29 +78,24 @@ func runMigrations(db *gorm.DB, driver string, zapLogger *zap.Logger) error {
 		return err
 	}
 
-	gooseDialect := "sqlite3"
-	migrationsSubDir := "sqlite"
-	if driver == "mysql" {
-		gooseDialect = "mysql"
-		migrationsSubDir = "mysql"
-	}
-
-	migrationsDir := getMigrationsDir(driver)
-	if _, err := os.Stat(migrationsDir); os.IsNotExist(err) {
-		return fmt.Errorf("迁移目录不存在: %s", migrationsDir)
+	dialect, fsys, err := migrations.ForDriver(driver)
+	if err != nil {
+		return err
 	}
 
 	if zapLogger != nil {
 		zapLogger.Info("执行数据库迁移",
-			zap.String("dialect", gooseDialect),
-			zap.String("dir", migrationsSubDir))
+			zap.String("dialect", string(dialect)),
+			zap.String("driver", driver))
 	}
 
-	if err := goose.SetDialect(gooseDialect); err != nil {
-		return err
+	provider, err := goose.NewProvider(dialect, sqlDB, fsys)
+	if err != nil {
+		return fmt.Errorf("创建迁移提供者失败: %w", err)
 	}
-	if err := goose.Up(sqlDB, migrationsDir); err != nil {
-		return err
+
+	if _, err := provider.Up(context.Background()); err != nil {
+		return fmt.Errorf("执行迁移失败: %w", err)
 	}
 
 	return nil
@@ -130,21 +126,6 @@ func configurePool(db *gorm.DB, cfg *config.DatabaseConfig, zapLogger *zap.Logge
 	}
 }
 
-func getMigrationsDir(driver string) string {
-	_, filename, _, ok := runtime.Caller(0)
-	if ok {
-		subDir := "sqlite"
-		if driver == "mysql" {
-			subDir = "mysql"
-		}
-		dir := filepath.Join(filepath.Dir(filename), "..", "..", "migrations", subDir)
-		if abs, err := filepath.Abs(dir); err == nil {
-			return abs
-		}
-	}
-	return "./migrations"
-}
-
 func BuildDSN(cfg *config.DatabaseConfig) string {
 	return fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=true&loc=Local",
 		cfg.User, cfg.Password, cfg.Host, cfg.Port, cfg.DBName)