feat(widgets): add unit/scale config to chart widget kinds

Graph widgets need consistent value scaling (kB/MB/GB, kbps/Mbps, …). Add
shared `unit` (none/bytes/bytes_per_sec/bits_per_sec/bits/percent/seconds) and
`scale` (auto/k/m/g/t) enum fields to:

- PrometheusChartWidgetConfig (alongside promql/window)
- new QbittorrentSpeedWidgetConfig — the speed widget previously had NO config
  options at all; totals/active keep their empty config.

Declared as Pydantic Literal enums so the widget-kind JSON schema exposes
`enum`, which the frontend config dialog renders as a dropdown. The data
sources are unchanged (raw values); scaling is a display concern handled
client-side. qBittorrent speed defaults to bytes/sec.

Test: chart widget kinds expose the shared unit/scale enums; speed defaults to
bytes_per_sec; totals/active stay option-less. 389/389 backend tests pass.

backend/tests/test_services.py

@@ -114,6 +114,29 @@ def test_widget_kind_lookup():
     assert get_widget_kind("unknown", "metric") is None
 
 
+def test_chart_widget_kinds_expose_unit_and_scale_options():
+    """Graph widgets share unit/scale options so axes/tooltips can be scaled."""
+    units = ["none", "bytes", "bytes_per_sec", "bits_per_sec", "bits", "percent", "seconds"]
+    scales = ["auto", "k", "m", "g", "t"]
+
+    prom_chart = get_widget_kind("prometheus", "chart")
+    assert prom_chart is not None
+    prom_props = prom_chart.config_schema["properties"]
+    assert prom_props["unit"]["enum"] == units
+    assert prom_props["scale"]["enum"] == scales
+
+    qbit_speed = get_widget_kind("qbittorrent", "speed")
+    assert qbit_speed is not None
+    qbit_props = qbit_speed.config_schema["properties"]
+    assert qbit_props["unit"]["enum"] == units
+    assert qbit_props["scale"]["enum"] == scales
+    # qBittorrent speed data is bytes/sec by default.
+    assert qbit_speed.default_config["unit"] == "bytes_per_sec"
+    # totals/active are not graphs and stay option-less.
+    assert "unit" not in get_widget_kind("qbittorrent", "totals").config_schema["properties"]
+    assert "unit" not in get_widget_kind("qbittorrent", "active").config_schema["properties"]
+
+
 def test_service_config_schema_is_json_schema():
     schema = get_service_definition("prometheus").config_schema
     assert schema["type"] == "object"