Allow editing an existing metric declaration in place (ticket 022)

Clicking a declared metric badge in Metrics.tsx now loads its regex/params
into the declaration form for editing (in addition to selecting it for the
bucket-values view), and shows those properties inline once selected rather
than only on hover. The backend gains PUT /api/metrics/{name}, backed by a
new engine::update_metric that reuses declare_metric's validation, requires
the metric to already exist, supports renaming, and re-backfills bucket data
from the new regex/params instead of leaving stale buckets from the old one.
Submitting the form while editing calls the update endpoint in place rather
than creating a second declaration; declaring a brand-new metric is unchanged.

Note: tickets/022-edit-existing-metric-declarations.md does not exist on this
worktree's branch (it was added to tickets/ on main after this branch was
created), so there is nothing to delete here.

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

webapp/src/lib/api.ts

@@ -200,6 +200,28 @@ async function postJson<T>(path: string, body: unknown): Promise<T> {
   return res.json() as Promise<T>
 }
 
+// Same error handling as `postJson` (throws `ApiError` with the parsed message) but for a PUT — used where updating in place, rather than creating, is the operation.
+async function putJson<T>(path: string, body: unknown): Promise<T> {
+  const res = await fetch(path, {
+    method: 'PUT',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  })
+  if (!res.ok) {
+    let message = `${path} failed: ${res.status}`
+    try {
+      const errorBody = (await res.json()) as Partial<ApiErrorBody>
+      if (errorBody && typeof errorBody.error === 'string') {
+        message = errorBody.error
+      }
+    } catch {
+      // response body wasn't JSON; fall back to the generic message
+    }
+    throw new ApiError(message)
+  }
+  return res.json() as Promise<T>
+}
+
 async function sendJson<T>(path: string, method: string, body?: unknown): Promise<T> {
   const res = await fetch(path, {
     method,
@@ -270,6 +292,11 @@ export function declareMetric(request: MetricDeclaration): Promise<MetricDeclara
   return postJson('/api/metrics', request)
 }
 
+// Updates the metric declaration named `existingName` in place (`existingName` may differ from `request.name` to rename it), rather than declaring a second metric or failing on a duplicate-name conflict.
+export function updateMetric(existingName: string, request: MetricDeclaration): Promise<MetricDeclaration> {
+  return putJson(`/api/metrics/${encodeURIComponent(existingName)}`, request)
+}
+
 export function fetchMetricBucketValues(
   name: string,
   params: {