Make delivery and integration failures explicit

Persist reminder presentations and retry state, atomically complete collapsed deliveries, fall back across away reaches, and block permanent failures visibly (V-715, V-678). Fail closed when enabled integrations lack credentials and keep remote arms explicitly dark (V-691). Give mavweb one sanitized, request-correlated error contract (V-689). Owner explicitly requested direct commits to master.

cmd/mavweb/reminders.html

@@ -5,7 +5,7 @@
 {{range .Reminders}}<tr>
 <td class=hint>{{.Created}}</td>
 <td>{{.Fires}}</td>
-<td><span class="badge {{.Status}}">{{.Status}}</span></td>
+<td><span class="badge {{.Status}}">{{.Status}}</span>{{if .Detail}}<div class=hint>{{.Detail}}</div>{{end}}</td>
 <td class=text-max>{{.Text}}</td>
 </tr>{{end}}</table></div>
 {{else}}<div class=empty>