feat(alerts): the metric catalog is written by what emits it (#5061)

#5061 "Metric catalog belongs to what emits: plugins register their
metrics; names match what is recorded".

- The static METRIC_CATALOG is gone. metric_catalog() is core's own sources
  (the four monitor types and "steward", the app's open_fds/open_fds_pct,
  which were recorded but never offered) plus each loaded plugin's
  get_alert_metrics() export. A disabled plugin's sources leave the picker.
- traefik offers what it records: error_rate_4xx_pct and error_rate_5xx_pct.
  The old "error_rate" was never recorded, so a rule on it could not fire.
- tests/core/test_metric_catalog.py walks every MetricBatch.add /
  record_metric call (traefik's tuple loop included) and fails on a recorded
  metric the catalog lacks; it shows on a synthetic writer that it can fail.

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

steward/alerts/routes.py

@@ -122,17 +122,17 @@ async def rule_fields_api():
 @alerts_bp.get("/rules/new")
 @require_role(UserRole.operator)
 async def new_rule():
-    first_source = alert_svc.SOURCE_MODULES[0]
+    first_source = alert_svc.source_modules()[0]
     async with current_app.db_sessionmaker() as db:
         resources = await alert_svc.resources_for(db, first_source)
     return await render_template(
         "alerts/rules_form.html",
         rule=None,
         operators=_OPERATORS,
-        source_modules=alert_svc.SOURCE_MODULES,
+        source_modules=alert_svc.source_modules(),
         initial_source=first_source,
         resources=resources,
-        metrics=alert_svc.METRIC_CATALOG.get(first_source, []),
+        metrics=alert_svc.metric_catalog().get(first_source, []),
         ansible_sources=_ansible_source_names(),
     )
 
@@ -150,10 +150,10 @@ async def edit_rule(rule_id: str):
         "alerts/rules_form.html",
         rule=rule,
         operators=_OPERATORS,
-        source_modules=alert_svc.SOURCE_MODULES,
+        source_modules=alert_svc.source_modules(),
         initial_source=rule.source_module,
         resources=resources,
-        metrics=alert_svc.METRIC_CATALOG.get(rule.source_module, []),
+        metrics=alert_svc.metric_catalog().get(rule.source_module, []),
         ansible_sources=_ansible_source_names(),
     )
 
@@ -256,7 +256,7 @@ async def list_maintenance():
         "alerts/maintenance.html",
         windows=windows,
         now=now,
-        source_modules=alert_svc.SOURCE_MODULES,
+        source_modules=alert_svc.source_modules(),
     )
 
 
@@ -266,7 +266,7 @@ async def new_maintenance():
     return await render_template(
         "alerts/maintenance_form.html",
         window=None,
-        source_modules=alert_svc.SOURCE_MODULES,
+        source_modules=alert_svc.source_modules(),
     )