feat(6254): moment_usage says which rule each moment delivered, and whether it was opened

Each moment gains by_rule: {rule_id, title, delivered, opened}, read from
the (moment, rule) grouping the readout already did. The moment's totals are
now summed from those rows, so the two cannot disagree. The Moments settings
page shows the list under each moment, closed by default.

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

frontend/src/components/MomentsSettings.vue

@@ -2,6 +2,7 @@
 import { computed, onMounted, ref } from "vue";
 import { useMomentsStore } from "@/stores/moments";
 import MomentProposals from "@/components/MomentProposals.vue";
+import MomentRuleUsage from "@/components/MomentRuleUsage.vue";
 import type { MomentAction, MomentUsage } from "@/api/moments";
 import { fmtDate } from "@/utils/dateFormat";
 
@@ -167,6 +168,7 @@ onMounted(() => store.load(true));
               >×</button>
             </span>
           </div>
+          <MomentRuleUsage v-if="usage(m.name)" :rules="usage(m.name)!.by_rule" />
         </li>
       </ul>
 
@@ -186,6 +188,7 @@ onMounted(() => store.load(true));
                 </template>
               </span>
             </div>
+            <MomentRuleUsage v-if="usage(name)" :rules="usage(name)!.by_rule" />
           </li>
         </ul>
       </template>