fix(scheduler): heartbeat at tick start + per-fire so liveness reflects work-in-progress

The first scheduler heartbeat (#95) only fired AFTER each tick completed.
A tick that runs fireSchedule for 110+ seconds (long agent prompts) would
make /admin/liveness report scheduler as stale even though it was actively
working. Observed today: scheduler firing UIUX audit, last_tick_at lagged
by 95s+ and incrementing.

Three places now call Heartbeat:
1. Top of tick() — proves we're past the ticker.C wait
2. Inside each fire goroutine, before fireSchedule — ANY active fire
   keeps the heartbeat fresh
3. Inside each fire goroutine, after fireSchedule — captures the moment
   the per-fire work completes

(The post-tick Heartbeat in Start() is still there as the "all idle" case.)

Net result: /admin/liveness reports stale only if the scheduler genuinely
isn't doing anything for >2× pollInterval, which is the actual signal we
want.

platform/internal/scheduler/scheduler.go

@@ -80,7 +80,16 @@ func (s *Scheduler) Start(ctx context.Context) {
 // tick queries all due schedules and fires each in a goroutine.
 // Waits for all goroutines to finish before returning so the next tick
 // doesn't re-fire schedules whose next_run_at hasn't been updated yet.
+//
+// Heartbeat is called at three points to keep /admin/liveness fresh during
+// long-running fires (some prompts take minutes — without these heartbeats
+// the scheduler looks "stale" the whole time it's working):
+//   - immediately on entering tick (proves we're past the ticker.C wait)
+//   - inside each per-fire goroutine (every fire bumps the heartbeat)
+//   - implicitly via the post-tick heartbeat in Start()
 func (s *Scheduler) tick(ctx context.Context) {
+	supervised.Heartbeat("scheduler")
+
 	rows, err := db.DB.QueryContext(ctx, `
 		SELECT id, workspace_id, name, cron_expr, timezone, prompt
 		FROM workspace_schedules
@@ -107,7 +116,15 @@ func (s *Scheduler) tick(ctx context.Context) {
 		go func(s2 scheduleRow) {
 			defer wg.Done()
 			defer func() { <-sem }()
+			defer func() {
+				if r := recover(); r != nil {
+					log.Printf("Scheduler: PANIC firing '%s' on workspace %s — recovered: %v",
+						s2.Name, s2.WorkspaceID, r)
+				}
+			}()
+			supervised.Heartbeat("scheduler")
 			s.fireSchedule(ctx, s2)
+			supervised.Heartbeat("scheduler")
 		}(sched)
 	}
 	if err := rows.Err(); err != nil {