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/src/media_library_viewer_api/integrations/qbittorrent.py

@@ -7,7 +7,7 @@ password), and three widget kinds (totals, active, speed). Models on
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Literal
 
 from media_library_viewer_api.clients.qbittorrent import QbittorrentClient
 from media_library_viewer_api.integrations.base import (
@@ -59,11 +59,26 @@ class QbittorrentConfig(ServiceConfigBase):
 
 
 class QbittorrentWidgetConfig(WidgetConfigBase):
-    """Per-widget config (empty — all three kinds derive from the service connection)."""
+    """Per-widget config for totals/active (empty — derived from the service connection)."""
 
     pass
 
 
+class QbittorrentSpeedWidgetConfig(WidgetConfigBase):
+    """Speed chart config. The source returns raw bytes/sec; the frontend scales."""
+
+    unit: Literal[
+        "none",
+        "bytes",
+        "bytes_per_sec",
+        "bits_per_sec",
+        "bits",
+        "percent",
+        "seconds",
+    ] = "bytes_per_sec"
+    scale: Literal["auto", "k", "m", "g", "t"] = "auto"
+
+
 DEFINITION = ServiceDefinition(
     service_type="qbittorrent",
     name="qBittorrent",
@@ -94,8 +109,8 @@ DEFINITION = ServiceDefinition(
             kind="speed",
             name="Speed chart",
             description="Live download/upload speed over a short window.",
-            model_cls=QbittorrentWidgetConfig,
-            default_config={},
+            model_cls=QbittorrentSpeedWidgetConfig,
+            default_config={"unit": "bytes_per_sec", "scale": "auto"},
             refresh_interval_ms=5_000,
         ),
     ],