feat(schedules): make schedules pausable

Adds pause/resume support for scheduled prompts.

Backend:
- New paused_at_unix column on scheduled_prompts table (added via
  ALTER TABLE migration so existing databases are upgraded on first
  start). The due-rows index is dropped and recreated to also exclude
  paused rows so the worker never fires them while paused.
- Worker's due() query gains AND paused_at_unix IS NULL filter.
- New pause(id) and resume(id) methods on ScheduledPrompts; both are
  idempotent and refuse cancelled rows.
- New POST /api/schedules/{id}/pause and /api/schedules/{id}/resume
  dashboard endpoints (operator-direct, no approval gate). Both emit
  a schedules snapshot on success so the tab updates live.
- WireSchedule gains paused_at_unix: Option<i64> so the frontend can
  render the state without an extra fetch.

Frontend:
- Paused rows render with a distinct row class + muted opacity.
- The next-fire cell shows a yellow pause glyph + tooltip with the
  paused-since timestamp and the would-have-fired time.
- Actions column: pause/resume toggle button (⏸/▶) beside fire/edit/cancel.
  Fire-now is disabled while paused (resume first).
- Sort order: active → paused → cancelled (paused slot keeps schedules
  visible without mixing them into the active top section).
- pauseSchedule() / resumeSchedule() async functions POST to the new
  endpoints and refresh the table on success.

frontend/packages/dashboard/src/dashboard.css

@@ -1100,6 +1100,10 @@ footer .banner-thin {
   opacity: 0.75;
 }
 .schedules-table-row-cancelled td { opacity: 0.55; }
+.schedules-table-row-paused td { opacity: 0.75; }
+.sched-paused-label { color: var(--yellow); font-size: 0.9em; }
+.btn-pause-schedule { color: var(--teal); }
+.btn-resume-schedule { color: var(--green); }
 .schedules-table-body-cell {
   max-width: 30em;
   overflow: hidden;