Replace Grafana iframe panel with server-side chart widget

The iframe-based 'panel' widget didn't work: the browser couldn't
authenticate against the OIDC-protected Grafana (Authentik), and
iframes can't carry Bearer tokens or share cross-origin session
cookies. Result: blank iframe or login redirect.

Replace it with a 'chart' widget that queries Grafana's datasource
API server-side:

Backend (GrafanaWidgetSource): POSTs to /api/ds/query with the stored
api_key (which bypasses OIDC), using the widget's configured PromQL
query, datasource_uid, time range, and resolution. Normalizes Grafana's
frame-based response into a simple {series: [{label, points: [{t, v}]}]}
shape. The api_key is never exposed to the browser.

Frontend (GrafanaChartWidget): renders the series data as a recharts
LineChart with dark-mode-aware colors (Tailwind --chart-* tokens),
responsive container, custom tooltip, and per-series lines. Loading
skeleton, error Alert, and empty state. recharts ^3.9.2 added.

The 'link' widget kind (deep-link URL) is unchanged. The 'panel' kind
and GrafanaPanelWidget are fully removed.

Backend: 279 tests pass (+1 net: -2 panel + 3 chart). Frontend: 127
tests pass (net 0: -3 panel + 3 chart). Lint/build green both sides.

backend/src/media_library_viewer_api/integrations/grafana.py

@@ -26,13 +26,15 @@ class GrafanaLinkWidgetConfig(WidgetConfigBase):
     panel_id: int | None = None
 
 
-class GrafanaPanelWidgetConfig(WidgetConfigBase):
-    """Embed a single Grafana panel via iframe."""
+class GrafanaChartWidgetConfig(WidgetConfigBase):
+    """Render a time-series chart from a Grafana datasource query."""
 
-    dashboard_uid: str
-    panel_id: int
+    datasource_uid: str = "prometheus"
+    query: str = ""
     from_ts: str = "now-1h"
     to_ts: str = "now"
+    interval_ms: int = 30_000
+    max_data_points: int = 100
 
 
 DEFINITION = ServiceDefinition(
@@ -53,12 +55,19 @@ DEFINITION = ServiceDefinition(
             refresh_interval_ms=0,
         ),
         widget_kind(
-            kind="panel",
-            name="Panel embed",
-            description="Embed a Grafana panel directly.",
-            model_cls=GrafanaPanelWidgetConfig,
-            default_config={"dashboard_uid": "", "panel_id": 1, "from_ts": "now-1h", "to_ts": "now"},
-            refresh_interval_ms=0,
+            kind="chart",
+            name="Chart",
+            description="Live time-series chart from a Grafana datasource query.",
+            model_cls=GrafanaChartWidgetConfig,
+            default_config={
+                "datasource_uid": "prometheus",
+                "query": "",
+                "from_ts": "now-1h",
+                "to_ts": "now",
+                "interval_ms": 30_000,
+                "max_data_points": 100,
+            },
+            refresh_interval_ms=60_000,
         ),
     ],
 )