Add atomic task deduplication with partial unique index

- Add partial unique index for active resource-scoped tasks
- Implement createTaskRunRecordAtomic for race-free task creation
- Update findOrEnqueueTask to use atomic insert first
- Add tests for concurrent task creation deduplication

lib/server/tasks.ts

@@ -7,6 +7,7 @@ import { describeTaskFailure } from '@/lib/server/task-errors';
 import {
   countTasksByStatus,
   createTaskRunRecord,
+  createTaskRunRecordAtomic,
   findInFlightTaskByResourceKey,
   getTaskByIdForUser,
   listTaskStageEventsForTask,
@@ -134,6 +135,38 @@ export async function findOrEnqueueTask(input: EnqueueTaskInput) {
     return await enqueueTask(input);
   }
 
+  const taskId = randomUUID();
+  const result = await createTaskRunRecordAtomic({
+    id: taskId,
+    user_id: input.userId,
+    task_type: input.taskType,
+    payload: input.payload ?? {},
+    priority: input.priority ?? 50,
+    max_attempts: input.maxAttempts ?? 3,
+    resource_key: input.resourceKey
+  });
+
+  if (result.created) {
+    try {
+      const run = await start(runTaskWorkflow, [result.task.id]);
+      await setTaskWorkflowRunId(result.task.id, run.runId);
+
+      return {
+        ...result.task,
+        workflow_run_id: run.runId
+      } satisfies Task;
+    } catch (error) {
+      const failure = describeTaskFailure(result.task, 'Failed to start workflow');
+      await markTaskFailure(result.task.id, failure.detail, 'failed', {
+        detail: failure.summary
+      });
+
+      const wrapped = new Error(failure.detail);
+      (wrapped as Error & { cause?: unknown }).cause = error;
+      throw wrapped;
+    }
+  }
+
   const existingTask = await findInFlightTaskByResourceKey(
     input.userId,
     input.taskType,
@@ -141,13 +174,10 @@ export async function findOrEnqueueTask(input: EnqueueTaskInput) {
   );
 
   if (existingTask) {
-    const reconciledTask = await reconcileTaskWithWorkflow(existingTask);
-    if (reconciledTask.status === 'queued' || reconciledTask.status === 'running') {
-      return reconciledTask;
-    }
+    return await reconcileTaskWithWorkflow(existingTask);
   }
 
-  return await enqueueTask(input);
+  throw new Error('Task deduplication conflict detected but no in-flight task found');
 }
 
 export async function findInFlightTask(userId: string, taskType: TaskType, resourceKey: string) {