Add DELETE /api/metrics/{name} to allow deleting a declared metric (ticket 051)

Wires the engine's existing RemoveMetricDeclaration message (previously
only used internally by delete_profile's cleanup) up to a new DELETE
route, persists the removal into the active profile so it survives a
restart, and adds a delete affordance in the Metrics UI.

Decisions:
- Bucket-value history under the deleted metric's name is purged, not
  left orphaned (engine.rs's new remove_metric_declaration reuses
  update_metric's clear_metric_data helper). This is what lets a
  dashboard panel or PromQL query still referencing the metric by name
  fail with a readable "unknown metric" error instead of silently
  returning stale data.
- evaluate_promql now errors when a metric has neither a declaration nor
  any bucket data, since a metric ingested via --metrics-url scraping
  never gets a MetricDeclaration but must stay queryable. Verified with
  a seeded fixture (promql_query_against_deleted_metric_fails_with_readable_error
  in src/api.rs).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/lib/api.ts

@@ -300,6 +300,10 @@ export function updateMetric(existingName: string, request: MetricDeclaration):
   return putJson(`/api/metrics/${encodeURIComponent(existingName)}`, request)
 }
 
+export function deleteMetric(name: string): Promise<void> {
+  return sendJson(`/api/metrics/${encodeURIComponent(name)}`, 'DELETE')
+}
+
 export function fetchMetricBucketValues(
   name: string,
   params: {