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>

steward/backup/contents.py

@@ -20,7 +20,7 @@ KEPT: frozenset[str] = frozenset({
     "alert_rules", "alert_states", "maintenance_windows",
     "ansible_targets", "ansible_groups", "ansible_target_groups", "ansible_schedules",
     "dashboards", "dashboard_widgets", "dashboard_share_tokens",
-    "snmp_devices", "prometheus_targets",
+    "snmp_devices", "prometheus_targets", "prometheus_widget_templates",
     # Names given to network clients by hand (UniFi's alias column).
     "unifi_known_clients",
     # History kept by the ruling.