fix(gpu): count backfill enqueues via RETURNING, not rowcount

result.rowcount is unreliable for INSERT…SELECT (returned -1), failing the
idempotency assert. Use .returning(GpuJob.id) and count the rows. (run 1652)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

backend/app/tasks/ml.py

@@ -760,10 +760,11 @@ def enqueue_gpu_backfill(task_name: str) -> int:
         sel = sa_select(
             ImageRecord.id, literal(task_name), literal("pending")
         ).where(~already)
-        result = session.execute(
-            insert(GpuJob).from_select(
-                ["image_record_id", "task", "status"], sel
-            )
-        )
+        # RETURNING + count: result.rowcount is unreliable for INSERT…SELECT.
+        rows = session.execute(
+            insert(GpuJob)
+            .from_select(["image_record_id", "task", "status"], sel)
+            .returning(GpuJob.id)
+        ).fetchall()
         session.commit()
-        return result.rowcount or 0
+        return len(rows)