feat(moments): the human door onto mounts, mappings and per-moment telemetry (milestone 458 step 6, #4924)

Everything an agent can do with moments, a person can now see and change
in the app.

- Rule editor: a moment picker beside the trigger. Catalog moments are
  ticked; a named procedure's `skill.<name>` is typed and checked as the
  server checks it. `moments` is always sent, so unticking the last moment
  unmounts the rule.
- Settings, Moments section (General tab): for each moment, what it
  means, the actions that reach it on this install (shipped ones can be
  switched off, the install's own removed), how many rules are mounted on
  it, and deliveries and agent opens over the window. Below that: named
  procedures with mounts, switched-off defaults with Restore, and a form to
  add an action.
- retrieval_telemetry.moment_usage: per moment, `delivered`, `rules`,
  `opened` (agent pulls after the first delivery there; an upper bound, as
  by_source is) and `last_delivered_at`. No ratio, because a mount is a
  person's statement, not a ranker's guess. Guarded on its own, and also
  reported in retrieval_summary as `moment_usage`.
- rulebooks.mount_counts; mounted_moments now derives from it.
- GET /api/retrieval/moments carries `mounted` and `usage` (?days=).
  DELETE /moments/mappings also reads the mapping from query parameters,
  since the browser's DELETE sends no body.

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

frontend/src/views/SettingsView.vue

@@ -8,6 +8,7 @@ import { apiGet, apiPost, apiPut, apiDelete, listGroups, createGroup, deleteGrou
 import type { User } from "@/types/auth";
 import PaginationBar from "@/components/PaginationBar.vue";
 import TagInput from "@/components/TagInput.vue";
+import MomentsSettings from "@/components/MomentsSettings.vue";
 import { fmtDate, fmtLogStamp } from "@/utils/dateFormat";
 import { fetchVersion, type VersionPayload } from "@/api/version";
 
@@ -2222,6 +2223,17 @@ async function deleteUser(userId: number) {
         </div>
       </section>
 
+      <section class="settings-section full-width">
+        <h2>Moments</h2>
+        <p class="section-desc">
+          A rule mounted on a moment arrives whenever that moment happens, whatever the work
+          looks like — which is how a rule about <em>when</em> something is done reaches a session
+          that never mentions it. Mount rules from the rule editor; here is what each moment
+          means, which actions reach it on this install, and what it has delivered.
+        </p>
+        <MomentsSettings />
+      </section>
+
     </div>
 
     <!-- ── Account ── -->