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_multi_device_collection.py

@@ -0,0 +1,90 @@
+import pytest
+
+from bambu_run.management.commands.bambu_collector import (
+    Command,
+    DeviceSession,
+    resolve_printer_device,
+)
+from bambu_run.models import PrintJob, PrinterMetrics
+
+
+class FakeClient:
+    """Stub in place of BambuPrinter — returns canned snapshots, no real MQTT."""
+
+    def __init__(self, snapshots):
+        self._snapshots = snapshots
+        self._index = 0
+        self._client = None  # cloud BambuClient handle used by cloud task sync
+
+    def get_snapshot(self):
+        snap = self._snapshots[min(self._index, len(self._snapshots) - 1)]
+        self._index += 1
+        return snap
+
+
+def make_session(device_id, name, snapshots):
+    printer = resolve_printer_device(device_id, {"name": name, "dev_product_name": "H2C"})
+    return DeviceSession(device_id=device_id, client=FakeClient(snapshots), printer=printer)
+
+
+@pytest.mark.django_db
+def test_collects_metrics_against_the_correct_printer_per_session():
+    session_a = make_session("SERIAL-A", "Printer A", [{"nozzle_temp": 200, "gcode_state": "IDLE"}])
+    session_b = make_session("SERIAL-B", "Printer B", [{"nozzle_temp": 210, "gcode_state": "IDLE"}])
+
+    cmd = Command()
+    cmd.verbose = False
+    cmd._collect_printer_data(session_a)
+    cmd._collect_printer_data(session_b)
+
+    metric_a = PrinterMetrics.objects.get(device=session_a.printer)
+    metric_b = PrinterMetrics.objects.get(device=session_b.printer)
+    assert metric_a.nozzle_temp == 200
+    assert metric_b.nozzle_temp == 210
+
+
+@pytest.mark.django_db
+def test_print_job_tracking_is_isolated_per_session():
+    session_a = make_session(
+        "SERIAL-A",
+        "Printer A",
+        [
+            {"gcode_state": "RUNNING", "subtask_name": "job_A", "print_percent": 10},
+            {"gcode_state": "FINISH", "subtask_name": "job_A", "print_percent": 100},
+        ],
+    )
+    session_b = make_session("SERIAL-B", "Printer B", [{"gcode_state": "IDLE"}])
+
+    cmd = Command()
+    cmd.verbose = False
+    cmd._collect_printer_data(session_a)
+    cmd._collect_printer_data(session_b)
+    cmd._collect_printer_data(session_a)
+
+    assert PrintJob.objects.filter(device=session_a.printer).count() == 1
+    job = PrintJob.objects.get(device=session_a.printer)
+    assert job.final_status == "FINISH"
+    assert session_a.current_print_job is None
+
+    assert PrintJob.objects.filter(device=session_b.printer).count() == 0
+    assert session_b.current_print_job is None
+
+
+@pytest.mark.django_db
+def test_one_session_error_does_not_affect_another_session():
+    session_a = make_session("SERIAL-A", "Printer A", [{"nozzle_temp": 200, "gcode_state": "IDLE"}])
+    session_b = make_session("SERIAL-B", "Printer B", [{"nozzle_temp": 210, "gcode_state": "IDLE"}])
+
+    class ExplodingClient:
+        def get_snapshot(self):
+            raise RuntimeError("MQTT connection lost")
+
+    session_a.client = ExplodingClient()
+
+    cmd = Command()
+    cmd.verbose = False
+    cmd._collect_printer_data(session_a)
+    cmd._collect_printer_data(session_b)
+
+    assert session_a.error_count == 1
+    assert PrinterMetrics.objects.filter(device=session_b.printer).exists()