fix: Delete completed tasks from pool instead of marking complete

Completed tasks are now deleted from worker_tasks table.
Only failed tasks remain in the pool for retry/review.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/tasks/task-service.ts

@@ -261,28 +261,24 @@ class TaskService {
   }
 
   /**
-   * Mark a task as completed with verification
-   * Returns true if completion was verified in DB, false otherwise
+   * Mark a task as completed and remove from pool
+   * Completed tasks are deleted - only failed tasks stay in the pool for retry/review
+   * Returns true if task was successfully deleted
    */
   async completeTask(taskId: number, result?: Record<string, unknown>): Promise<boolean> {
-    await pool.query(
-      `UPDATE worker_tasks
-       SET status = 'completed', completed_at = NOW(), result = $2, error_message = NULL
-       WHERE id = $1`,
-      [taskId, result ? JSON.stringify(result) : null]
-    );
-
-    // Verify completion was recorded
-    const verify = await pool.query(
-      `SELECT status FROM worker_tasks WHERE id = $1`,
+    // Delete the completed task from the pool
+    // Only failed tasks stay in the table for retry/review
+    const deleteResult = await pool.query(
+      `DELETE FROM worker_tasks WHERE id = $1 RETURNING id`,
       [taskId]
     );
 
-    if (verify.rows[0]?.status !== 'completed') {
-      console.error(`[TaskService] Task ${taskId} completion NOT VERIFIED - DB shows status: ${verify.rows[0]?.status}`);
+    if (deleteResult.rowCount === 0) {
+      console.error(`[TaskService] Task ${taskId} completion FAILED - task not found or already deleted`);
       return false;
     }
 
+    console.log(`[TaskService] Task ${taskId} completed and removed from pool`);
     return true;
   }