feat(prometheus-direct-charting): slice 1 — prom range query + chart rebrand

Add PrometheusWidgetSource._fetch_chart hitting /api/v1/query_range directly
(SC-101..104). New shared helpers in widgets/prometheus_range.py:
step_for_window (window preset -> step, ~200pts) and normalize_prometheus_matrix
(extracted label/dedup rule, retargeted at Prom matrix, robust to malformed
data). Chart widget kind moved grafana->prometheus in both registries;
GrafanaChartWidget renamed -> PrometheusChartWidget (git mv, recharts body
preserved). Grafana binding/service untouched (removed in slice 3).

Backend: 298 pytest pass, ruff clean. Frontend: 128 vitest pass, build+lint green.

frontend/src/integrations/registry.test.ts

@@ -23,6 +23,9 @@ describe("service registry", () => {
 	it("binds widget kinds per service", () => {
 		expect(SERVICE_REGISTRY.grafana.widgets.map((w) => w.kind)).toEqual([
 			"link",
+		]);
+		expect(SERVICE_REGISTRY.prometheus.widgets.map((w) => w.kind)).toEqual([
+			"metric",
 			"chart",
 		]);
 		expect(SERVICE_REGISTRY.alertmanager.widgets.map((w) => w.kind)).toEqual([