diff --git a/backend/internal/app/admin_storage_delete_test.go b/backend/internal/app/admin_storage_delete_test.go
index 82f331a31..d829d8cc5 100644
--- a/backend/internal/app/admin_storage_delete_test.go
+++ b/backend/internal/app/admin_storage_delete_test.go
@@ -10,7 +10,6 @@ import (
 	"infinite-canvas/backend/internal/model"
 	"infinite-canvas/backend/internal/repository"
 
-	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
 )
 
@@ -162,10 +161,7 @@ func TestDeleteAdminResourcesRejectsNonAdminAndOversizedBatch(t *testing.T) {
 
 func newAdminStorageDeleteTestService(t *testing.T) (*Service, *gorm.DB, string, *model.User) {
 	t.Helper()
-	db, err := gorm.Open(sqlite.Open("file:"+newID()+"?mode=memory&cache=shared"), &gorm.Config{})
-	if err != nil {
-		t.Fatal(err)
-	}
+	db := newSQLiteTestDB(t)
 	if err := database.MigrateSchema(db); err != nil {
 		t.Fatal(err)
 	}
diff --git a/backend/internal/app/resource_delete_test.go b/backend/internal/app/resource_delete_test.go
index 7c541bde1..b0a640477 100644
--- a/backend/internal/app/resource_delete_test.go
+++ b/backend/internal/app/resource_delete_test.go
@@ -11,7 +11,6 @@ import (
 	"infinite-canvas/backend/internal/model"
 	"infinite-canvas/backend/internal/repository"
 
-	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
 )
 
@@ -509,10 +508,7 @@ func TestDetachedFailedResourceWithoutObjectKeyNeedsNoDeletionJob(t *testing.T)
 
 func newResourceDeletionTestService(t *testing.T) (*Service, *gorm.DB, string) {
 	t.Helper()
-	db, err := gorm.Open(sqlite.Open("file:"+newID()+"?mode=memory&cache=shared"), &gorm.Config{})
-	if err != nil {
-		t.Fatal(err)
-	}
+	db := newSQLiteTestDB(t)
 	if err := database.MigrateSchema(db); err != nil {
 		t.Fatal(err)
 	}
diff --git a/backend/internal/app/test_database_test.go b/backend/internal/app/test_database_test.go
new file mode 100644
index 000000000..55f71bc37
--- /dev/null
+++ b/backend/internal/app/test_database_test.go
@@ -0,0 +1,100 @@
+package app
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+// newSQLiteTestDB 打开一个测试用的 SQLite 库。
+//
+// 默认写法 `file:<id>?mode=memory&cache=shared` 在"后台 worker 并发写 + 测试线程读"时会报
+// `database table is locked`：共享缓存模式下 SQLite 用的是**表级锁**（SQLITE_LOCKED），
+// 它不受 `_busy_timeout` 约束（那只管 SQLITE_BUSY），只能靠 `sqlite3_unlock_notify` 重试，
+// 而当前驱动没有启用。实测并发读写 400 次能撞到 70+ 次，CI 上因此偶发失败
+// （TestDeleteGeneratedAssetTaskReferences/cancelled_output 就是这样挂的）。
+//
+// 因此凡是会触发后台删除 outbox（`go s.drainResourceDeletionJobs`）的用例，统一用
+// 文件库 + WAL + busy_timeout：WAL 允许"一写多读"并发，写冲突退化成 SQLITE_BUSY 并由
+// busy_timeout 重试。
+//
+// 库文件刻意放在 `os.MkdirTemp` 而不是 `t.TempDir()` 下：outbox goroutine 可能在用例返回后
+// 仍在写库，`t.TempDir()` 的清理遇到这种写入会以 "directory not empty" 直接判失败，
+// 而它本身与用例断言无关。
+func newSQLiteTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+	directory, err := os.MkdirTemp("", "canvas-test-db-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = os.RemoveAll(directory) })
+	db, err := gorm.Open(sqlite.Open(filepath.Join(directory, "test.db")+"?_journal_mode=WAL&_busy_timeout=5000"), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return db
+}
+
+// TestSQLiteTestDBAllowsConcurrentReadWrite 守的是 newSQLiteTestDB 的选型：并发读写不得报锁。
+// 换回 `mode=memory&cache=shared` 会立刻变红（见上面的机制说明）。
+func TestSQLiteTestDBAllowsConcurrentReadWrite(t *testing.T) {
+	db := newSQLiteTestDB(t)
+	if err := db.Exec("create table lock_probe (id integer primary key, value text)").Error; err != nil {
+		t.Fatal(err)
+	}
+	if err := db.Exec("insert into lock_probe (id, value) values (1, 'a')").Error; err != nil {
+		t.Fatal(err)
+	}
+
+	var mutex sync.Mutex
+	var failure error
+	record := func(err error) {
+		if err == nil {
+			return
+		}
+		mutex.Lock()
+		defer mutex.Unlock()
+		if failure == nil {
+			failure = err
+		}
+	}
+
+	stop := make(chan struct{})
+	var workers sync.WaitGroup
+	workers.Add(2)
+	go func() {
+		defer workers.Done()
+		for {
+			select {
+			case <-stop:
+				return
+			default:
+			}
+			var count int64
+			record(db.Table("lock_probe").Count(&count).Error)
+		}
+	}()
+	go func() {
+		defer workers.Done()
+		defer close(stop)
+		for index := 0; index < 300; index++ {
+			record(db.Exec("update lock_probe set value = ? where id = 1", fmt.Sprint(index)).Error)
+			time.Sleep(time.Millisecond)
+		}
+	}()
+	workers.Wait()
+
+	if failure != nil {
+		if strings.Contains(failure.Error(), "locked") {
+			t.Fatalf("并发读写命中 SQLite 锁：%v（说明测试库不该回退到共享缓存内存库）", failure)
+		}
+		t.Fatalf("并发读写失败：%v", failure)
+	}
+}