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/migrations/versions/prometheus_002_templates.py

@@ -0,0 +1,33 @@
+"""Widget templates an admin writes or copies in are rows
+
+Each row is one template's YAML and its name, beside the bundled templates
+that ship as files (#5692).
+
+Revision ID: prometheus_002_templates
+Revises: prometheus_001_targets
+Create Date: 2026-10-14
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "prometheus_002_templates"
+down_revision: Union[str, None] = "prometheus_001_targets"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "prometheus_widget_templates",
+        sa.Column("id", sa.String(36), primary_key=True),
+        sa.Column("name", sa.String(80), nullable=False, unique=True),
+        sa.Column("text", sa.Text, nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("prometheus_widget_templates")