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/__init__.py

@@ -11,7 +11,7 @@ _app: "Quart | None" = None
 def setup(app: "Quart") -> None:
     global _app
     _app = app
-    from .models import PrometheusTarget  # noqa: F401 — registers the table
+    from .models import PrometheusTarget, PrometheusWidgetTemplate  # noqa: F401 — registers the tables
     register_widgets()
 
 
@@ -64,6 +64,7 @@ def get_alert_metrics() -> dict[str, list[str]]:
 
 
 def get_audit_entities() -> dict:
-    """Audit rows about a target name it and link to its page."""
-    from .models import PrometheusTarget
-    return {"prometheus_target": (PrometheusTarget, "/plugins/prometheus/targets/{}")}
+    """Audit rows about a target or a widget template name it and link to its page."""
+    from .models import PrometheusTarget, PrometheusWidgetTemplate
+    return {"prometheus_target": (PrometheusTarget, "/plugins/prometheus/targets/{}"),
+            "prometheus_template": (PrometheusWidgetTemplate, "/plugins/prometheus/templates/custom:{}")}