fix(vision): reference image sopravvive al copy-on-write (#38.6)

_copy_tasks_to_version non copiava le righe VisionReferenceImage del
task sorgente: il task copiato perdeva le immagini di riferimento del
grafo di visione. Ora copia le righe sul task nuovo, stesso path e
stessi metadati (stesso file su disco, non duplicato).
Di conseguenza il file va cancellato solo se nessun'altra riga
VisionReferenceImage referenzia lo stesso path: nuova
vision_service.file_still_referenced(), usata dalla route DELETE
prima di remove_reference_file.
Test: copy-on-write elenca l'immagine sulla nuova versione; DELETE di
una delle due righe lascia il file; DELETE dell'ultima lo rimuove.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/tests/test_vision_reference.py

@@ -192,6 +192,78 @@ async def test_deleting_an_unknown_reference_image_is_404(
     assert response.status_code == 404
 
 
+@pytest.mark.asyncio
+async def test_copy_on_write_carries_over_reference_images(
+    client, db_session, maker_user, measurement_tec_user,
+):
+    """#38.6: a copy-on-write version must list the source task's reference
+    images (same path, same metadata), and the file is removed only once no
+    row references it any more."""
+    recipe = await create_test_recipe(db_session, maker_user.id)
+    version, task, subtask = await recipe_parts(db_session, recipe.id)
+    await db_session.commit()
+
+    created = await client.post(
+        "/api/vision/reference-images",
+        headers=auth_headers(maker_user),
+        files={"image": ("ref.png", an_image(), "image/png")},
+        data={"task_id": str(task.id), "note": "orig"},
+    )
+    assert created.status_code == 200
+    old_reference_id = created.json()["id"]
+    path = created.json()["path"]
+    file_path = settings.upload_path / path
+    assert file_path.exists()
+
+    # A measurement on v1 forces the next PUT onto the copy-on-write path.
+    measured = await client.post(
+        "/api/measurements/",
+        headers=auth_headers(measurement_tec_user),
+        json={"subtask_id": subtask.id, "version_id": version.id, "value": 10.0},
+    )
+    assert measured.status_code == 200
+
+    resp = await client.put(
+        f"/api/recipes/{recipe.id}",
+        headers=auth_headers(maker_user),
+        json={"change_notes": "Edit after measurement"},
+    )
+    assert resp.status_code == 200, resp.text
+
+    db_session.expunge_all()
+    new_version, new_task, _ = await recipe_parts(db_session, recipe.id)
+    assert new_version.version_number == 2
+
+    listing = await client.get(
+        "/api/vision/reference-images",
+        headers=auth_headers(maker_user),
+        params={"task_id": new_task.id},
+    )
+    assert listing.status_code == 200
+    body = listing.json()
+    assert len(body) == 1
+    assert body[0]["path"] == path
+    assert body[0]["note"] == "orig"
+    new_reference_id = body[0]["id"]
+    assert new_reference_id != old_reference_id
+
+    # Two rows share the same file: deleting one must leave it in place.
+    del1 = await client.delete(
+        f"/api/vision/reference-images/{old_reference_id}",
+        headers=auth_headers(maker_user),
+    )
+    assert del1.status_code == 204
+    assert file_path.exists()
+
+    # Deleting the last row referencing the path removes the file too.
+    del2 = await client.delete(
+        f"/api/vision/reference-images/{new_reference_id}",
+        headers=auth_headers(maker_user),
+    )
+    assert del2.status_code == 204
+    assert not file_path.exists()
+
+
 @pytest.mark.asyncio
 async def test_preview_runs_the_graph_and_saves_no_measurement(
     client, db_session, maker_user, worker_says,