Feature/multi printer support (#12)

* Initial implementation of multi-printer support.

* Always show device dropdown and add bambu_diagnose for multi-printer troubleshooting.

* Add multi-AMS support: per-unit snapshot/usage tracking, grouped dashboard panels with real type labels, and dual-nozzle card UX fixes. Fixes a real-world AMS info-code parsing bug found by inspecting live H2C data.

* Add Vortek hotend rack tracking: per-SN registry with slot mapping confirmed against live MQTT capture, plus a fallback for non-inductive nozzles (e.g. H2C's fixed left nozzle) shown read-only without fabricated identity. New dashboard card hides entirely on printers with no Vortek/nozzle-info data at all.

tests/test_printer_routing.py

@@ -0,0 +1,78 @@
+import pytest
+from django.urls import reverse
+
+from bambu_run.models import Printer
+
+
+@pytest.fixture
+def logged_in_client(client, django_user_model):
+    user = django_user_model.objects.create_user(username="tester", password="pw")
+    client.force_login(user)
+    return client
+
+
+@pytest.mark.django_db
+def test_dashboard_with_no_printers_shows_error(logged_in_client):
+    resp = logged_in_client.get(reverse("bambu_run:printer_dashboard"))
+    assert resp.status_code == 200
+    assert "error" in resp.context
+
+
+@pytest.mark.django_db
+def test_dashboard_defaults_to_first_active_printer(logged_in_client):
+    printer = Printer.objects.create(name="Only Printer", model="H2C", is_active=True)
+
+    resp = logged_in_client.get(reverse("bambu_run:printer_dashboard"))
+
+    assert resp.context["printer_device"].pk == printer.pk
+    assert list(resp.context["all_printers"]) == [printer]
+    # Switcher shows even with a single printer, as a hint that multi-printer exists.
+    assert resp.context["show_printer_switcher"] is True
+
+
+@pytest.mark.django_db
+def test_dashboard_hides_switcher_with_zero_printers(logged_in_client):
+    resp = logged_in_client.get(reverse("bambu_run:printer_dashboard"))
+
+    assert resp.context["show_printer_switcher"] is False
+
+
+@pytest.mark.django_db
+def test_dashboard_pk_route_shows_requested_printer(logged_in_client):
+    Printer.objects.create(name="Printer A", model="H2C", is_active=True)
+    printer_b = Printer.objects.create(name="Printer B", model="X1C", is_active=True)
+
+    resp = logged_in_client.get(
+        reverse("bambu_run:printer_dashboard", kwargs={"pk": printer_b.pk})
+    )
+
+    assert resp.context["printer_device"].pk == printer_b.pk
+    assert resp.context["device_name"] == "Printer B"
+
+
+@pytest.mark.django_db
+def test_dashboard_unknown_pk_returns_404(logged_in_client):
+    resp = logged_in_client.get(
+        reverse("bambu_run:printer_dashboard", kwargs={"pk": 99999})
+    )
+    assert resp.status_code == 404
+
+
+@pytest.mark.django_db
+def test_api_pk_route_returns_only_requested_printer_data(logged_in_client):
+    from bambu_run.models import PrinterMetrics
+    from django.utils import timezone
+    from decimal import Decimal
+
+    printer_a = Printer.objects.create(name="Printer A", model="H2C", is_active=True)
+    printer_b = Printer.objects.create(name="Printer B", model="X1C", is_active=True)
+    PrinterMetrics.objects.create(device=printer_a, timestamp=timezone.now(), nozzle_temp=Decimal("200"))
+    PrinterMetrics.objects.create(device=printer_b, timestamp=timezone.now(), nozzle_temp=Decimal("210"))
+
+    resp = logged_in_client.get(
+        reverse("bambu_run:printer_api", kwargs={"pk": printer_b.pk})
+    )
+
+    assert resp.status_code == 200
+    data = resp.json()
+    assert data["nozzle_temp"] == [210.0]