Add pause and cancel to a running migration

A live run could only be stopped one account at a time, and stopping it
at all meant losing the queue: the accounts that had not started yet
stayed idle with no record that they were meant to run.

A run now carries a handle holding the context that stops every account
under it plus the reason it was stopped. Pause and cancel take the same
path and differ only in the status left behind — paused accounts are
what Resume re-runs, and the migration journal makes each one continue
where it stopped instead of re-copying. Accounts still queued when the
stop lands get the same status as the interrupted ones, so the whole
remainder is resumable after a pause and cancelled after a cancel.
Database writes keep using the uncancellable context, so statuses and
counters survive the stop.

The scheduler skips paused tasks: auto-starting a full run would defeat
the pause. An operator stopping a run no longer trips the schedule
breaker either — that is for failures, not for intent.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/store/tasks.go

@@ -120,13 +120,16 @@ type SchedulableTask struct {
 }
 
 // ListSchedulableTasks returns tasks eligible to auto-run: schedule on, not
-// broken, not currently running — each joined with its last finished run time.
+// broken, neither running nor paused — each joined with its last finished run
+// time. A paused task waits for the operator to resume it; auto-starting a full
+// run behind their back would defeat the pause.
 func (s *Store) ListSchedulableTasks(ctx context.Context) ([]SchedulableTask, error) {
 	rows, err := s.Pool.Query(ctx,
 		`SELECT t.id, t.schedule_interval_seconds, t.schedule_anchor,
 		        (SELECT max(finished_at) FROM runs r WHERE r.task_id=t.id AND r.finished_at IS NOT NULL)
 		 FROM tasks t
-		 WHERE t.schedule_interval_seconds > 0 AND NOT t.broken AND t.status <> 'running'`)
+		 WHERE t.schedule_interval_seconds > 0 AND NOT t.broken
+		   AND t.status <> 'running' AND t.status <> 'paused'`)
 	if err != nil {
 		return nil, err
 	}