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>

steward/dashboard/routes.py

@@ -3,7 +3,7 @@ import hashlib
 import json
 import secrets
 from datetime import datetime, timezone, timedelta
-from urllib.parse import urlencode
+from urllib.parse import quote, urlencode
 from quart import Blueprint, render_template, current_app, abort, redirect, request, session
 from sqlalchemy import select, func, update
 from steward.auth.middleware import public_route, require_role, current_user_id
@@ -127,13 +127,41 @@ def _resolve_widgets(widgets: list[DashboardWidget]) -> list[dict]:
         hx_url = defn["hx_url"] + "?" + urlencode(url_params)
         out.append({
             "db": w,
-            "defn": defn,
+            "defn": {**defn, "detail_url": detail_url(defn, config)},
             "hx_url": hx_url,
             "title": w.title or defn["label"],
         })
     return out
 
 
+def detail_url(defn: dict, config: dict) -> str:
+    """The widget's Details link. A plugin widget may name its settings in
+    it, "/plugins/prometheus/targets/{target}"; when one isn't set, the link
+    goes to the section above, "/plugins/prometheus/"."""
+    url = defn["detail_url"]
+    if "{" not in url:
+        return url
+    try:
+        return url.format(**{k: quote(str(v), safe="") for k, v in config.items()})
+    except (KeyError, IndexError, ValueError):
+        return url[:url.index("{")].rsplit("/", 2)[0] + "/"
+
+
+async def with_options(db, widget: dict) -> dict:
+    """A widget definition for the picker, its `options_from` settings
+    filled in now: a plugin's targets, templates and the like, read when
+    the picker opens rather than fixed in the registry."""
+    params = widget.get("params", [])
+    if not any(p.get("options_from") for p in params):
+        return widget
+    filled = []
+    for p in params:
+        if p.get("options_from"):
+            p = {**p, "options": await p["options_from"](db)}
+        filled.append(p)
+    return {**widget, "params": filled}
+
+
 # ── Edit panel helpers ────────────────────────────────────────────────────────
 
 # ── Root redirect ─────────────────────────────────────────────────────────────
@@ -173,7 +201,7 @@ async def view(dash_id: int):
         # viewers/shared-readers don't, so skip the extra queries for them.
         if can_edit:
             plugins_cfg = current_app.config.get("PLUGINS", {})
-            addable = get_available_widgets(plugins_cfg)
+            addable = [await with_options(db, w) for w in get_available_widgets(plugins_cfg)]
             hosts   = list((await db.execute(select(Host).order_by(Host.name))).scalars())
         else:
             addable, hosts = [], []
@@ -339,6 +367,8 @@ async def add_widget(dash_id: int, widget_key: str):
                 config[p["key"]] = val
     config_json = json.dumps(config) if config else None
     async with current_app.db_sessionmaker() as db:
+        if title is None and defn.get("default_title"):
+            title = await defn["default_title"](db, config)
         async with db.begin():
             result = await db.execute(select(Dashboard).where(Dashboard.id == dash_id))
             dash   = result.scalar_one_or_none()