feat(prometheus): the widget template library: list, edit with live preview, paste in, upload, export, duplicate (#5692)

Prometheus gains a "Widget templates" tab beside Targets. It lists the
built-in templates and an admin's own, any that no longer pass the checker
first. A template opens as its YAML beside a live preview drawn over a
chosen target exactly as the dashboard widget draws it (routes.draw, now
shared by both). An admin writes or pastes a new one, reads a .yaml upload
into the editor to look over, duplicates any template under a free name,
edits and deletes their own; anyone can export the YAML. Built-in
templates are read-only.

Custom templates are rows in prometheus_widget_templates (migration
prometheus_002_templates), kept in backups, keyed "custom:<id>" so a
rename keeps its widgets. Saving needs a template that passes the checker
and a name no other template has. Create, update and delete are audited
and link to the template.

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

plugins/prometheus/templates/prometheus/form.html

@@ -5,6 +5,8 @@
 {% extends "base.html" %}
 {% from "_macros.html" import crumbs, page_header %}
 {% block title %}{{ ("Edit " ~ target.name) if target else "Add Prometheus target" }} — Steward{% endblock %}
+{% from "prometheus/_tabs.html" import prometheus_tabs %}
+{% block section_tabs %}{{ prometheus_tabs("targets") }}{% endblock %}
 {% block breadcrumb %}
 {%- if target -%}
 {{ crumbs([("Prometheus", "/plugins/prometheus/"), (target.name, "/plugins/prometheus/targets/" ~ target.id), ("Edit", "")]) }}