feat(prometheus): widget template format, checker and renderer (#5690)

#5690 "Widget template format: YAML schema, validator and renderer", step 2
of milestone 508 "Prometheus widget templates".

- template_format.check(text) -> Template, or TemplateError with every
  problem, each with its line and what to change. Blocks: states, values
  (of -> share and bar, thresholds as numbers or shares, sparkline),
  times (warn_after / crit_after), table (two labels pivoted), info.
  Anchors/aliases refused, 64 KB cap, YAML's on/off/yes/no flagged.
- template_view.render(template, target, latest, now, history) -> View:
  a verdict naming the worst problem, then each block with its problems
  first (crit, warn, ok, then quiet states). Missing series show as "—"
  and say so; a template whose `expects` prefix the target lacks says so.
- shown_value moves to service.py so the renderer can use it.

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

plugins/prometheus/routes.py

@@ -13,7 +13,6 @@ from sqlalchemy import delete, func, select, update
 from steward.auth.middleware import require_role
 from steward.core.audit import log_audit
 from steward.core.crypto import decrypt_secret
-from steward.core.formatting import metric_value
 from steward.core.metric_series import latest_values, points
 from steward.models.metrics import PluginMetric, PluginMetricLatest
 from steward.models.users import UserRole
@@ -21,7 +20,7 @@ from steward.models.users import UserRole
 from .models import PrometheusTarget
 from .scraper import ScrapeError, scrape
 from .service import (OWN_METRICS, SOURCE, FormError, fold_states, form_values, map_families,
-                      parse_form, patterns, target_state)
+                      parse_form, patterns, shown_value, target_state)
 
 prometheus_bp = Blueprint("prometheus", __name__, template_folder="templates")
 
@@ -48,19 +47,6 @@ def view(target: PrometheusTarget, own: dict, now: datetime) -> dict:
             "scrape_ms": own.get("scrape_ms")}
 
 
-def shown_value(metric: str, value: float | None) -> dict:
-    """A series' value in its unit, from the Prometheus naming conventions.
-    A *_timestamp_seconds is a moment: it comes back as `moment` for the page
-    to show as "3m ago" in the reader's zone; 0 there means "never"."""
-    if value is None:
-        return {"text": "—", "moment": None}
-    if metric.endswith("_timestamp_seconds"):
-        if value <= 0:
-            return {"text": "never", "moment": None}
-        return {"text": "", "moment": datetime.fromtimestamp(value, timezone.utc)}
-    return {"text": metric_value(metric, value), "moment": None}
-
-
 @prometheus_bp.get("/")
 @require_role(UserRole.viewer)
 async def index():