reminders: add recurring reminder support

Add cron expression support for recurring reminders using robfig/cron/v3.

Changes:
- Migration #2: ALTER TABLE reminders ADD COLUMN cron TEXT + next_fire_ts INTEGER
- Reminder struct: add Cron and NextFireTs fields
- scanReminder helper extracts full row including nullable cron
- CreateReminder: accept optional cron param, store next_fire_ts = fire_ts
- DueReminders: query on next_fire_ts instead of fire_ts
- RescheduleReminder: new method — parse cron, compute next fire, update
  next_fire_ts or mark fired if no more valid times
- Dispatcher: call RescheduleReminder for cron reminders, MarkReminder for
  one-shots (preserving existing behavior for ID=0 digest skip)
- ReminderCompleter interface: add RescheduleReminder method
- storeAPI adapter: forward RescheduleReminder
- All callers updated: CreateReminder signature includes cron param
- Tests: TestRecurringReminder (store), TestDispatchRecurringReminderReschedules
- Existing tests updated for new signature

internal/ipc/server.go

@@ -68,8 +68,8 @@ func (a *storeAPI) Presence(ctx context.Context) (Presence, error) {
 	return Presence{Bucket: Bucket(b), Score: score, Updated: upd}, nil
 }
 
-func (a *storeAPI) CreateReminder(ctx context.Context, fire time.Time, payload string) (int64, error) {
-	id, err := a.s.CreateReminder(ctx, fire, payload)
+func (a *storeAPI) CreateReminder(ctx context.Context, fire time.Time, payload, cron string) (int64, error) {
+	id, err := a.s.CreateReminder(ctx, fire, payload, cron)
 	return id, mapErr(err)
 }
 
@@ -77,6 +77,10 @@ func (a *storeAPI) MarkReminder(ctx context.Context, id int64, status string) er
 	return mapErr(a.s.MarkReminder(ctx, id, status))
 }
 
+func (a *storeAPI) RescheduleReminder(ctx context.Context, id int64, now time.Time) error {
+	return mapErr(a.s.RescheduleReminder(ctx, id, now))
+}
+
 func (a *storeAPI) RecordNudge(ctx context.Context, rule, channel, message string, ts time.Time) (int64, error) {
 	id, err := a.s.RecordNudge(ctx, rule, channel, message, ts)
 	return id, mapErr(err)
@@ -457,7 +461,7 @@ func (s *Server) dispatch(ctx context.Context, req Request) (json.RawMessage, er
 		if err := unmarshalParams(req.Params, &p); err != nil {
 			return nil, err
 		}
-		id, err := s.api.CreateReminder(ctx, p.Fire, p.Payload)
+		id, err := s.api.CreateReminder(ctx, p.Fire, p.Payload, p.Cron)
 		return marshalResult(idResp{ID: id}), err
 
 	case MethodMarkReminder: