fix(vision): file non immagine e grafo malformato non escono più come 500 (#15)

Worker: Image.open().convert() in try/except (422 "image is not
decodable"); il grafo deve essere un dict (422 altrimenti); run_graph
distingue costruzione (GraphBuildError -> 422) da esecuzione (500 con
motivo). Router: verifica dei magic bytes (PNG/JPEG/BMP/TIFF/WebP/GIF)
prima di chiamare il worker.

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

src/backend/tests/test_vision_upload_limits.py

@@ -102,6 +102,32 @@ async def test_execute_rejects_a_non_image_upload(
     assert (await db_session.execute(select(Measurement))).scalars().all() == []
 
 
+@pytest.mark.asyncio
+async def test_execute_rejects_a_file_whose_bytes_do_not_match_its_declared_type(
+    client, db_session, measurement_tec_user, worker_says,
+):
+    """#15: a text file declared image/png passes the Content-Type check but
+    must still not reach the worker - it is refused on its magic bytes."""
+    recipe = await create_test_recipe(db_session, measurement_tec_user.id)
+    version, task, subtask = await recipe_parts(db_session, recipe.id)
+    task.task_type = "camera_measure"
+    task.vision_json = {"schema_version": 1, "name": "p", "tools": []}
+    subtask.vision_output = "t1.diameter"
+    await db_session.commit()
+    worker_says({"t1.diameter": float(subtask.nominal)})
+
+    response = await client.post(
+        "/api/vision/execute",
+        headers=auth_headers(measurement_tec_user),
+        files={"image": ("p.png", b"i am not an image", "image/png")},
+        data={"task_id": str(task.id)},
+    )
+
+    assert response.status_code == 422
+    assert (await db_session.execute(select(VisionResult))).scalars().all() == []
+    assert (await db_session.execute(select(Measurement))).scalars().all() == []
+
+
 @pytest.mark.asyncio
 async def test_execute_rejects_an_oversized_upload(
     client, db_session, measurement_tec_user, worker_says, monkeypatch,