Fix cross-model queue items being misrepresented and editable into a broken state (#671)

The edit dialog offered a printer picker and target-model dropdown for
an item with alternatives. Saving left a row with variants AND a
printer_id, and the scheduler's fixed-printer branch wins that race, so
it dispatched a row whose library_file_id is still null and failed in
the upload. PATCH now refuses printer/model changes on such an item —
comparing against the current value, since the dialog re-sends
target_model unchanged — and the route eager-loads variants, without
which the guard could not see them and every PATCH response dropped the
alternatives from its payload.

Names come from a shared helper now. A cross-model item holds neither
archive_id nor library_file_id until dispatch, so five separate inlined
fallbacks all rendered "File #null"; they now read "x1c.gcode.3mf +1
more".

The queue also grouped these under "Any H2D" — the first candidate
mirrored onto the row — filing a job under a printer it might never run
on. It groups as "Any H2D / X1C", matching the row beneath it.

backend/tests/integration/test_queue_variants_api.py

@@ -222,6 +222,58 @@ class TestQueueWithVariants:
         assert r.status_code == 400
         assert "No active printers" in r.json()["detail"]
 
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_assigning_a_printer_is_refused(self, async_client, db_session, sliced_file_factory, printer_factory):
+        """The edit dialog offers a printer picker for every queue item. Taking it
+        would leave a row with variants AND a printer_id — and the fixed-printer
+        branch of the scheduler wins that race, dispatching a row whose
+        library_file_id is still null."""
+        printer = await printer_factory(model="H2S")
+        await printer_factory(model="H2C")
+        h2s = await sliced_file_factory("H2S")
+        h2c = await sliced_file_factory("H2C")
+        item_id = (await _queue_variants(async_client, h2s.id, h2c.id)).json()["id"]
+
+        r = await async_client.patch(f"/api/v1/queue/{item_id}", json={"printer_id": printer.id})
+        assert r.status_code == 400
+        assert "alternatives" in r.json()["detail"]
+
+        assert len(await _variants_of(db_session, item_id)) == 2, "the alternatives survive the refusal"
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_narrowing_to_one_model_is_refused(self, async_client, sliced_file_factory, printer_factory):
+        """Saving "Any H2C" over a two-candidate job would silently discard the
+        H2S alternative the user deliberately queued."""
+        await printer_factory(model="H2S")
+        await printer_factory(model="H2C")
+        h2s = await sliced_file_factory("H2S")
+        h2c = await sliced_file_factory("H2C")
+        item_id = (await _queue_variants(async_client, h2s.id, h2c.id)).json()["id"]
+
+        r = await async_client.patch(f"/api/v1/queue/{item_id}", json={"target_model": "H2C"})
+        assert r.status_code == 400
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_resending_the_unchanged_model_is_allowed(self, async_client, sliced_file_factory, printer_factory):
+        """The edit dialog re-sends target_model on every save, so an unchanged
+        value must not block editing the schedule or print options."""
+        await printer_factory(model="H2S")
+        await printer_factory(model="H2C")
+        h2s = await sliced_file_factory("H2S")
+        h2c = await sliced_file_factory("H2C")
+        created = (await _queue_variants(async_client, h2s.id, h2c.id)).json()
+
+        r = await async_client.patch(
+            f"/api/v1/queue/{created['id']}",
+            json={"target_model": created["target_model"], "timelapse": True},
+        )
+        assert r.status_code == 200
+        assert r.json()["timelapse"] is True
+        assert len(r.json()["variants"]) == 2, "the response still carries the alternatives"
+
     @pytest.mark.asyncio
     @pytest.mark.integration
     async def test_quantity_gives_each_copy_its_own_candidates(