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/embedded_migration_test.go

@@ -0,0 +1,71 @@
+package database
+
+import (
+	"io/fs"
+	"testing"
+
+	"nex/backend/migrations"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEmbeddedMigrations_SQLiteResourcesPresent(t *testing.T) {
+	entries, err := fs.ReadDir(migrations.FS, "sqlite")
+	require.NoError(t, err)
+
+	var sqlFiles []string
+	for _, entry := range entries {
+		if !entry.IsDir() {
+			sqlFiles = append(sqlFiles, entry.Name())
+		}
+	}
+	assert.NotEmpty(t, sqlFiles, "SQLite 迁移资源应至少包含一个 .sql 文件")
+}
+
+func TestEmbeddedMigrations_MySQLResourcesPresent(t *testing.T) {
+	entries, err := fs.ReadDir(migrations.FS, "mysql")
+	require.NoError(t, err)
+
+	var sqlFiles []string
+	for _, entry := range entries {
+		if !entry.IsDir() {
+			sqlFiles = append(sqlFiles, entry.Name())
+		}
+	}
+	assert.NotEmpty(t, sqlFiles, "MySQL 迁移资源应至少包含一个 .sql 文件")
+}
+
+func TestEmbeddedMigrations_SQLiteSQLParsable(t *testing.T) {
+	subFS, err := fs.Sub(migrations.FS, "sqlite")
+	require.NoError(t, err)
+
+	entries, err := fs.ReadDir(subFS, ".")
+	require.NoError(t, err)
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+		data, err := fs.ReadFile(subFS, entry.Name())
+		require.NoError(t, err, "无法读取迁移文件: %s", entry.Name())
+		assert.NotEmpty(t, data, "迁移文件内容不应为空: %s", entry.Name())
+	}
+}
+
+func TestEmbeddedMigrations_MySQLSQLParsable(t *testing.T) {
+	subFS, err := fs.Sub(migrations.FS, "mysql")
+	require.NoError(t, err)
+
+	entries, err := fs.ReadDir(subFS, ".")
+	require.NoError(t, err)
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+		data, err := fs.ReadFile(subFS, entry.Name())
+		require.NoError(t, err, "无法读取迁移文件: %s", entry.Name())
+		assert.NotEmpty(t, data, "迁移文件内容不应为空: %s", entry.Name())
+	}
+}