fix(vp): show target printer's serial on proxy-mode VP card

The runtime services (SSDP, MQTT bind identity, cert subject) already
  advertise the target printer's serial via target_printer_serial or
  self.serial in proxy mode, but the API response that drives the VP
  settings card always returned the self-generated suffix-based serial.
  The card therefore displayed a serial that didn't match what slicers
  see, breaking the "one identity per VP" mental model.

  _vp_to_dict now resolves vp.target_printer_id -> Printer.serial_number
  when mode == VP_MODE_PROXY and substitutes the result into the response
  serial field. Archive / queue / review keep the self-generated serial
  (those modes never speak the target's identity). Orphaned target falls
  back to self-generated so the card still renders.

backend/tests/integration/test_virtual_printer_api.py

@@ -599,3 +599,83 @@ class TestVirtualPrinterAccessCodeInheritance:
 
         vp = (await db_session.execute(select(VirtualPrinter).where(VirtualPrinter.id == vp_id))).scalar_one()
         assert vp.access_code == "REALCODE"
+
+
+class TestVirtualPrinterSerialSurface:
+    """Proxy-mode VPs must surface the target printer's serial in API responses.
+
+    The bridge advertises the target's serial over SSDP and forwards the
+    target's identity to the slicer; the VP-settings card should show the
+    same serial so the user sees one consistent identity per VP. Archive /
+    queue / review VPs keep the self-generated suffix-based serial since
+    those modes never speak the target's identity.
+    """
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_proxy_vp_response_uses_target_printer_serial(self, async_client: AsyncClient, printer_factory):
+        target = await printer_factory(name="Real X1C", access_code="REALCODE", serial_number="00M09A123456789")
+
+        create_resp = await async_client.post(
+            "/api/v1/virtual-printers",
+            json={
+                "name": "ProxyVP",
+                "mode": "proxy",
+                "target_printer_id": target.id,
+            },
+        )
+        assert create_resp.status_code == 200
+        vp_id = create_resp.json()["id"]
+        assert create_resp.json()["serial"] == "00M09A123456789"
+
+        get_resp = await async_client.get(f"/api/v1/virtual-printers/{vp_id}")
+        assert get_resp.status_code == 200
+        assert get_resp.json()["serial"] == "00M09A123456789"
+
+        list_resp = await async_client.get("/api/v1/virtual-printers")
+        assert list_resp.status_code == 200
+        listed = next(p for p in list_resp.json()["printers"] if p["id"] == vp_id)
+        assert listed["serial"] == "00M09A123456789"
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_non_proxy_vp_keeps_self_generated_serial(self, async_client: AsyncClient, printer_factory):
+        # Even with a target printer set (#1429 access-code inheritance flow),
+        # archive / queue / review VPs are NOT bridging the target's identity
+        # to the slicer — they synthesise their own. Self-generated serial.
+        target = await printer_factory(name="Real X1C", access_code="REALCODE", serial_number="00M09A123456789")
+
+        create_resp = await async_client.post(
+            "/api/v1/virtual-printers",
+            json={
+                "name": "QueueVP",
+                "mode": "queue",
+                "target_printer_id": target.id,
+            },
+        )
+        assert create_resp.status_code == 200
+        assert create_resp.json()["serial"] != "00M09A123456789"
+        # The synthesised serial follows _get_serial_for_model's `<prefix><suffix>`
+        # shape — model-specific prefix + 8-char hex suffix from `vp.serial_suffix`.
+        assert len(create_resp.json()["serial"]) >= 8
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_proxy_vp_falls_back_to_self_generated_when_target_missing(
+        self, async_client: AsyncClient, db_session
+    ):
+        # Defensive fallback: a proxy VP whose target_printer_id points at a
+        # row that no longer exists (printer deleted mid-config, manual SQL
+        # tweak, race) must not 500 — it returns the self-generated serial
+        # so the card still renders and the user can fix the target.
+        from backend.app.models.virtual_printer import VirtualPrinter
+
+        vp = VirtualPrinter(name="OrphanProxy", mode="proxy", target_printer_id=99999, enabled=False)
+        db_session.add(vp)
+        await db_session.commit()
+
+        get_resp = await async_client.get(f"/api/v1/virtual-printers/{vp.id}")
+        assert get_resp.status_code == 200
+        body = get_resp.json()
+        assert body["serial"]  # non-empty
+        assert len(body["serial"]) >= 8