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.

backend/src/media_library_viewer_api/widgets/sources.py

@@ -12,6 +12,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+import time
 from dataclasses import dataclass, field
 from typing import Any, Protocol
 
@@ -25,6 +26,11 @@ from media_library_viewer_api.domain.dashboard import (
 from media_library_viewer_api.integrations.alertmanager import summarize_alerts
 from media_library_viewer_api.services.settings_store import SettingsStore, get_settings_store
 from media_library_viewer_api.services.task_runner import run_saved_task
+from media_library_viewer_api.widgets.prometheus_range import (
+    WINDOW_PRESETS,
+    normalize_prometheus_matrix,
+    step_for_window,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -209,7 +215,7 @@ class GrafanaWidgetSource:
 
 
 class PrometheusWidgetSource:
-    """Run a PromQL instant query against a Prometheus service."""
+    """Run PromQL queries against a Prometheus service (instant + range)."""
 
     async def fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) -> dict[str, Any]:
         try:
@@ -217,6 +223,9 @@ class PrometheusWidgetSource:
                 return {"error": "Prometheus widget is missing its service"}
             base_url = str(service.config.get("base_url") or "").rstrip("/")
             timeout = int(service.config.get("timeout_seconds") or 10)
+            if widget_kind == "chart":
+                return await self._fetch_chart(base_url, timeout, config)
+            # Default: instant-query metric path (unchanged).
             promql = config.get("promql")
             if not promql:
                 return {"error": "promql is required"}
@@ -242,6 +251,47 @@ class PrometheusWidgetSource:
             logger.exception("prometheus adapter failed")
             return {"error": f"Prometheus query failed: {exc}"}
 
+    async def _range_query(self, base_url: str, timeout: int, promql: str, window: int) -> dict[str, Any]:
+        """Run a Prometheus ``/api/v1/query_range`` over a window (seconds).
+
+        Shared by the ``chart`` (SC-101) and ``mean`` widget kinds. Returns
+        ``{"matrix": result}`` on success or ``{"error": str}`` (never raises,
+        per SC-103).
+        """
+        step = step_for_window(window)
+        end = int(time.time())
+        start = end - window
+        try:
+            response = await asyncio.wait_for(
+                asyncio.to_thread(
+                    requests.get,
+                    f"{base_url}/api/v1/query_range",
+                    params={"query": promql, "start": start, "end": end, "step": step},
+                    timeout=timeout,
+                ),
+                timeout=timeout,
+            )
+            response.raise_for_status()
+            payload = response.json()
+        except asyncio.TimeoutError:
+            return {"error": "Prometheus query timed out"}
+        except requests.RequestException as exc:
+            logger.exception("prometheus range query failed")
+            return {"error": f"Prometheus query failed: {exc}"}
+        result = payload.get("data", {}).get("result", [])
+        return {"matrix": result}
+
+    async def _fetch_chart(self, base_url: str, timeout: int, config: dict[str, Any]) -> dict[str, Any]:
+        """Range query → ``{series}`` for the chart widget (SC-101..SC-104)."""
+        promql = config.get("promql")
+        if not promql:
+            return {"error": "promql is required"}
+        window = WINDOW_PRESETS.get(config.get("window", "1h"), WINDOW_PRESETS["1h"])
+        raw = await self._range_query(base_url, timeout, promql, window)
+        if "error" in raw:
+            return raw
+        return {"series": normalize_prometheus_matrix(raw["matrix"])}
+
 
 class AlertmanagerWidgetSource:
     """Fetch firing alerts from an Alertmanager service and summarize them."""