feat: live SSE updates for the SCH3DUL3S tab

Add `SchedulesChanged` to the dashboard event channel so the
operator's schedule list updates in real time without requiring a
tab-activation or form-submit refresh.

Backend:
- `dashboard_events.rs`: new `SchedulesChanged { seq, schedules }`
  variant carrying a full `Vec<WireSchedule>` snapshot (same
  snapshot-over-diff rationale as `RebuildQueueChanged`).
- `coordinator.rs`: `emit_schedules_snapshot()` helper — queries the
  scheduled_prompts list, converts to wire shape, broadcasts the event.
- `dashboard.rs`: call `emit_schedules_snapshot()` at the end of each
  operator API handler that mutates a schedule:
  `post_schedule_new`, `post_schedule_fire_now`,
  `patch_schedule`, `post_schedule_cancel`.
- `scheduled_prompts_worker.rs`: call `emit_schedules_snapshot()`
  after each tick that fires schedules, so `last_fired_at_unix`,
  `next_fire_at_unix`, and reaped one-shots surface live.

Frontend:
- `tabs.js`: add `applySchedulesChanged(ev)` — replaces
  `schedulesState` from the snapshot and calls `renderSchedulesList()`.
  Registered in `MUTATION_HANDLERS` as `schedules_changed`.
  Tab-activation re-fetch kept as safety net for approval-path
  inserts and disconnect windows; comment updated to reflect this.

Docs:
- `docs/web-ui/dashboard.md`: document `schedules_changed` event.
- `CLAUDE.md`: add `SchedulesChanged` to the file-map entry.

hive-c0re/src/scheduled_prompts_worker.rs

@@ -69,6 +69,9 @@ fn tick(coord: &Arc<Coordinator>) {
     if let Err(e) = coord.scheduled_prompts.reap_cancelled(cutoff) {
         tracing::warn!(error = ?e, "scheduled_prompts: reap cancelled failed");
     }
+    // Emit after all fires + reaps so the dashboard reflects updated
+    // last_fired_at_unix, next_fire_at_unix, and any reaped one-shots.
+    coord.emit_schedules_snapshot();
 }
 
 /// Fan out one schedule's body to every active target. Records