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.

docs/web-ui/dashboard.md

@@ -843,6 +843,15 @@ payload):
   `meta_inputs_changed`: the list is small and the client's
   `parent_id` grouping is most naturally re-derived from the
   full list. Cold-loaded from `/api/state.rebuild_queue`.
+- `schedules_changed` (seq, schedules: `Vec<WireSchedule>`) —
+  full snapshot of all scheduled prompts. Emitted after every
+  operator mutation via the `/api/schedules` surface (new /
+  edit / cancel / fire-now) and after the worker fires or
+  rearms a row. Same snapshot-shape rationale as
+  `rebuild_queue_changed`. The SCH3DUL3S tab subscribes and
+  re-renders `schedulesState` on receipt; tab activation still
+  re-fetches as a safety net for approval-path inserts and
+  disconnect windows.
 
 `/api/state` is **only fetched on cold-load and on the few
 forms that mutate non-event-derived state** (PURG3 +