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/library.py

@@ -0,0 +1,71 @@
+"""The widget templates a Prometheus widget can draw with.
+
+Bundled templates ship in plugins/prometheus/widgets/*.yaml, read once and
+read-only; their key is "bundled:<file name>". Templates an admin copies in
+join them with the template library (#5692). Steward fetches none from
+anywhere (System 181's ruling).
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from functools import lru_cache
+from pathlib import Path
+
+from sqlalchemy import select
+
+from .models import PrometheusTarget
+from .template_format import Problem, Template, TemplateError, check
+
+BUNDLED = Path(__file__).parent / "widgets"
+
+
+@dataclass
+class Entry:
+    key: str
+    name: str
+    bundled: bool
+    text: str
+    template: Template | None = None
+    problems: list[Problem] = field(default_factory=list)
+
+
+def _entry(key: str, fallback_name: str, text: str, bundled: bool) -> Entry:
+    try:
+        template = check(text)
+    except TemplateError as exc:
+        return Entry(key, fallback_name, bundled, text, None, exc.problems)
+    return Entry(key, template.name, bundled, text, template)
+
+
+@lru_cache(maxsize=1)
+def bundled() -> tuple[Entry, ...]:
+    return tuple(_entry(f"bundled:{p.stem}", p.stem, p.read_text(encoding="utf-8"), True)
+                 for p in sorted(BUNDLED.glob("*.yaml")))
+
+
+async def entries(db) -> list[Entry]:
+    """Every template, by name."""
+    return sorted(bundled(), key=lambda e: e.name.lower())
+
+
+async def find(db, key: str) -> Entry | None:
+    return next((e for e in await entries(db) if e.key == key), None)
+
+
+# ── The widget's settings ────────────────────────────────────────────────────
+
+async def target_options(db) -> list[dict]:
+    rows = (await db.execute(select(PrometheusTarget.id, PrometheusTarget.name)
+                             .order_by(PrometheusTarget.name))).all()
+    return [{"value": tid, "label": name} for tid, name in rows]
+
+
+async def template_options(db) -> list[dict]:
+    return [{"value": e.key, "label": f"{e.name} (built in)" if e.bundled else e.name}
+            for e in await entries(db) if e.template is not None]
+
+
+async def default_title(db, config: dict) -> str | None:
+    """A new widget is named after its template: "Fabled Archives"."""
+    entry = await find(db, str(config.get("template") or ""))
+    return entry.name if entry and entry.template else None