fix: anchor floor on re-enable; breaker on scheduled panic; propagate next-run err; reject negative interval; close phantom runs on startup

internal/store/runs.go

@@ -56,6 +56,8 @@ func (s *Store) ListRunsByTask(ctx context.Context, taskID int64) ([]Run, error)
 
 // LastFinishedRunAt returns the most recent finished run's timestamp, or nil if
 // the task has never completed a run — the baseline for the next scheduled run.
+// The same max(finished_at) rule is also inlined in ListSchedulableTasks's
+// subquery — keep the two in sync if this changes.
 func (s *Store) LastFinishedRunAt(ctx context.Context, taskID int64) (*time.Time, error) {
 	var t *time.Time
 	err := s.Pool.QueryRow(ctx,