fix(coderd): clean up task snapshots on task deletion (#21949)

Task snapshots were orphaned when tasks were soft-deleted. The
`task_snapshots` table has an `ON DELETE CASCADE` foreign key, but
that only fires on hard deletes.

Modified DeleteTask to use a CTE that atomically soft-deletes the
task and removes its snapshot in a single transaction. The query now
returns just the task UUID instead of the full row.

Closes coder/internal#1283

coderd/database/queries/tasks.sql

@@ -57,13 +57,19 @@ AND CASE WHEN @status::text != '' THEN tws.status = @status::task_status ELSE TR
 ORDER BY tws.created_at DESC;
 
 -- name: DeleteTask :one
-UPDATE tasks
-SET
-	deleted_at = @deleted_at::timestamptz
-WHERE
-	id = @id::uuid
-	AND deleted_at IS NULL
-RETURNING *;
+WITH deleted_task AS (
+	UPDATE tasks
+	SET
+		deleted_at = @deleted_at::timestamptz
+	WHERE
+		id = @id::uuid
+		AND deleted_at IS NULL
+	RETURNING id
+), deleted_snapshot AS (
+	DELETE FROM task_snapshots
+	WHERE task_id = @id::uuid
+)
+SELECT id FROM deleted_task;
 
 
 -- name: UpdateTaskPrompt :one