refactor(数据库): 迁移持久层到 Ent 并清理 GORM

将仓储层/基础设施改为 Ent + 原生 SQL 执行路径，并移除 AutoMigrate 与 GORM 依赖。
重构内容包括：
- 仓储层改用 Ent/SQL（含 usage_log/account 等复杂查询），统一错误映射
- 基础设施与 setup 初始化切换为 Ent + SQL migrations
- 集成测试与 fixtures 迁移到 Ent 事务模型
- 清理遗留 GORM 模型/依赖，补充迁移与文档说明
- 增加根目录 Makefile 便于前后端编译

测试：
- go test -tags unit ./...
- go test -tags integration ./...

backend/internal/repository/soft_delete_ent_integration_test.go

@@ -0,0 +1,110 @@
+//go:build integration
+
+package repository
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"testing"
+
+	dbent "github.com/Wei-Shaw/sub2api/ent"
+	"github.com/Wei-Shaw/sub2api/ent/apikey"
+	"github.com/Wei-Shaw/sub2api/ent/schema/mixins"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+func uniqueSoftDeleteValue(t *testing.T, prefix string) string {
+	t.Helper()
+	safeName := strings.NewReplacer("/", "_", " ", "_").Replace(t.Name())
+	return fmt.Sprintf("%s-%s", prefix, safeName)
+}
+
+func createEntUser(t *testing.T, ctx context.Context, client *dbent.Client, email string) *dbent.User {
+	t.Helper()
+
+	u, err := client.User.Create().
+		SetEmail(email).
+		SetPasswordHash("test-password-hash").
+		Save(ctx)
+	require.NoError(t, err, "create ent user")
+	return u
+}
+
+func TestEntSoftDelete_ApiKey_DefaultFilterAndSkip(t *testing.T) {
+	ctx := context.Background()
+	client, _ := testEntSQLTx(t)
+
+	u := createEntUser(t, ctx, client, uniqueSoftDeleteValue(t, "sd-user")+"@example.com")
+
+	repo := NewApiKeyRepository(client)
+	key := &service.ApiKey{
+		UserID: u.ID,
+		Key:    uniqueSoftDeleteValue(t, "sk-soft-delete"),
+		Name:   "soft-delete",
+		Status: service.StatusActive,
+	}
+	require.NoError(t, repo.Create(ctx, key), "create api key")
+
+	require.NoError(t, repo.Delete(ctx, key.ID), "soft delete api key")
+
+	_, err := repo.GetByID(ctx, key.ID)
+	require.ErrorIs(t, err, service.ErrApiKeyNotFound, "deleted rows should be hidden by default")
+
+	_, err = client.ApiKey.Query().Where(apikey.IDEQ(key.ID)).Only(ctx)
+	require.Error(t, err, "default ent query should not see soft-deleted rows")
+	require.True(t, dbent.IsNotFound(err), "expected ent not-found after default soft delete filter")
+
+	got, err := client.ApiKey.Query().
+		Where(apikey.IDEQ(key.ID)).
+		Only(mixins.SkipSoftDelete(ctx))
+	require.NoError(t, err, "SkipSoftDelete should include soft-deleted rows")
+	require.NotNil(t, got.DeletedAt, "deleted_at should be set after soft delete")
+}
+
+func TestEntSoftDelete_ApiKey_DeleteIdempotent(t *testing.T) {
+	ctx := context.Background()
+	client, _ := testEntSQLTx(t)
+
+	u := createEntUser(t, ctx, client, uniqueSoftDeleteValue(t, "sd-user2")+"@example.com")
+
+	repo := NewApiKeyRepository(client)
+	key := &service.ApiKey{
+		UserID: u.ID,
+		Key:    uniqueSoftDeleteValue(t, "sk-soft-delete2"),
+		Name:   "soft-delete2",
+		Status: service.StatusActive,
+	}
+	require.NoError(t, repo.Create(ctx, key), "create api key")
+
+	require.NoError(t, repo.Delete(ctx, key.ID), "first delete")
+	require.NoError(t, repo.Delete(ctx, key.ID), "second delete should be idempotent")
+}
+
+func TestEntSoftDelete_ApiKey_HardDeleteViaSkipSoftDelete(t *testing.T) {
+	ctx := context.Background()
+	client, _ := testEntSQLTx(t)
+
+	u := createEntUser(t, ctx, client, uniqueSoftDeleteValue(t, "sd-user3")+"@example.com")
+
+	repo := NewApiKeyRepository(client)
+	key := &service.ApiKey{
+		UserID: u.ID,
+		Key:    uniqueSoftDeleteValue(t, "sk-soft-delete3"),
+		Name:   "soft-delete3",
+		Status: service.StatusActive,
+	}
+	require.NoError(t, repo.Create(ctx, key), "create api key")
+
+	require.NoError(t, repo.Delete(ctx, key.ID), "soft delete api key")
+
+	// Hard delete using SkipSoftDelete so the hook doesn't convert it to update-deleted_at.
+	_, err := client.ApiKey.Delete().Where(apikey.IDEQ(key.ID)).Exec(mixins.SkipSoftDelete(ctx))
+	require.NoError(t, err, "hard delete")
+
+	_, err = client.ApiKey.Query().
+		Where(apikey.IDEQ(key.ID)).
+		Only(mixins.SkipSoftDelete(ctx))
+	require.True(t, dbent.IsNotFound(err), "expected row to be hard deleted")
+}