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/__init__.py

@@ -12,6 +12,34 @@ def setup(app: "Quart") -> None:
     global _app
     _app = app
     from .models import PrometheusTarget  # noqa: F401 — registers the table
+    register_widgets()
+
+
+def register_widgets() -> None:
+    """One dashboard widget for every target and template (System 181's
+    ruling). Its target and template lists are read when the widget picker
+    opens, so a new target or template appears without a restart."""
+    from steward.core.widgets import register_widget
+
+    from . import library
+    register_widget({
+        "key": "prometheus",
+        "label": "Prometheus",
+        "description": "One scrape target drawn by a widget template: Fabled Archives, or one you copy in",
+        "hx_url": "/plugins/prometheus/widget",
+        "detail_url": "/plugins/prometheus/targets/{target}",
+        "plugin": "prometheus",
+        "poll": True,
+        "default_title": library.default_title,
+        "params": [
+            {"key": "target", "label": "Target", "type": "select", "default": "",
+             "options_from": library.target_options,
+             "empty": "No targets yet: add one under Prometheus first."},
+            {"key": "template", "label": "Template", "type": "select", "default": "",
+             "options_from": library.template_options,
+             "empty": "No templates yet."},
+        ],
+    })
 
 
 def get_scheduled_tasks() -> list: