feat(prometheus): the Prometheus dashboard widget, target + template (#5691)

#5691 "Generic Prometheus dashboard widget: pick a target and a template",
step 3 of milestone 508 "Prometheus widget templates".

- /plugins/prometheus/widget draws one target with one template in widget
  shape: the verdict (led by "isn't answering" when the target is down),
  then each block with problems first, figures beside a share bar or the
  last 24h of bars, and the healthy rest folding to "+N more healthy" past
  six. A missing target or template, or one that no longer checks, says so.
- library.py: bundled templates from plugins/prometheus/widgets/*.yaml,
  the picker's target and template lists, and the default title.
- widgets/fabled_archives.yaml: the first bundled template, against the
  series Archives reports (refined in #5693).
- Dashboard: a widget setting may take `options_from` (read when the picker
  opens) with an `empty` sentence; `default_title`; a Details link may name
  settings ("/plugins/prometheus/targets/{target}"). Prometheus is grouped
  with the integrations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/prometheus/routes.py

@@ -308,3 +308,57 @@ async def series_detail(target_id: str):
         "prometheus/series.html", target=target, resource=resource, metric=metric, labels=labels,
         now=shown_value(metric, now_row.value if now_row else None), hours=hours, ranges=RANGES,
         points_json=json.dumps([(t.isoformat(), v) for t, v in series]), stamp_axis=stamp_axis)
+
+
+# ── The dashboard widget ─────────────────────────────────────────────────────
+
+SPARK_HOURS = 24
+
+
+def spark_heights(values: list[float]) -> list[float]:
+    """Bar heights in % of the series' own peak over the window (System
+    154's ruling on small history bars); a flat zero draws flat."""
+    peak = max((abs(v) for v in values), default=0.0)
+    return [round(100.0 * abs(v) / peak, 1) if peak else 0.0 for v in values]
+
+
+@prometheus_bp.get("/widget")
+@require_role(UserRole.viewer)
+async def widget():
+    """The Prometheus dashboard widget: one target drawn by one template
+    (System 181's ruling), in widget shape. Reads stored values only."""
+    from . import library
+    from .template_view import render as draw
+
+    target_id = request.args.get("target") or ""
+    key = request.args.get("template") or ""
+    now = datetime.now(timezone.utc)
+    async with current_app.db_sessionmaker() as db:
+        target = await db.get(PrometheusTarget, target_id) if target_id else None
+        entry = await library.find(db, key) if key else None
+        problem = ("This widget has no target; remove it and add it again with one." if not target_id
+                   else "The target this widget showed has been deleted." if target is None
+                   else "This widget has no template; remove it and add it again with one." if not key
+                   else "The template this widget used is gone." if entry is None
+                   else f"The template “{entry.name}” has a problem: {entry.problems[0]}"
+                   if entry.template is None else "")
+        if problem:
+            return await render_template("prometheus/widget.html", problem=problem, target=None,
+                                         view=None)
+        latest = await latest_values(db, SOURCE, target.name, include_children=True)
+        view = draw(entry.template, target.name, latest, now)
+        if view.wants:
+            since = now - timedelta(hours=SPARK_HOURS)
+            history = {}
+            for resource, metric in view.wants:
+                got = await points(db, SOURCE, resource, metric, since, target=SPARK_HOURS)
+                history[(resource, metric)] = spark_heights([v for _, v in got])
+            view = draw(entry.template, target.name, latest, now, history=history)
+    return await render_template("prometheus/widget.html", problem="", view=view,
+                                 target=view_target(target, now), spark_hours=SPARK_HOURS)
+
+
+def view_target(target: PrometheusTarget, now: datetime) -> dict:
+    state = target_state(target, now)
+    return {"id": target.id, "name": target.name, "reach": state.reach, "says": state.says,
+            "ok_at": target.last_ok_at, "scraped_at": target.last_scraped_at}