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.

hive-c0re/src/socket_server.rs

@@ -2021,6 +2021,7 @@ fn schedule_to_wire(s: crate::scheduled_prompts::Schedule) -> hive_sh4re::WireSc
             }
         },
         cancelled_at_unix: s.cancelled_at_unix,
+        paused_at_unix: s.paused_at_unix,
         description: s.description,
         targets: s
             .targets
@@ -2126,6 +2127,7 @@ mod tests {
             created_at_unix: 0,
             source: hive_sh4re::WireScheduleSource::Operator,
             cancelled_at_unix: None,
+            paused_at_unix: None,
             description: None,
             targets: targets.iter().map(|t| target(t)).collect(),
         }