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

cmd/mavend/tick_test.go

@@ -150,7 +150,7 @@ func TestTickReminderFiresOnceAndMarkedFired(t *testing.T) {
 	st := newTestStore(t)
 	ctx := context.Background()
 	now := refNow()
-	if _, err := st.CreateReminder(ctx, now.Add(-time.Minute), `{"text":"stand up"}`); err != nil {
+	if _, err := st.CreateReminder(ctx, now.Add(-time.Minute), `{"text":"stand up"}`, ""); err != nil {
 		t.Fatalf("CreateReminder: %v", err)
 	}
 	sink := &fakeSink{}

cmd/mavend/voice.go

@@ -354,7 +354,7 @@ func (h *reactiveHandler) applyAction(ctx context.Context, dec router.Decision)
 			return "не получилось разобрать время напоминания."
 		}
 		payload := `{"text":` + jsonString(dec.Utterance) + `}`
-		if _, err := h.api.CreateReminder(ctx, dec.Slots.Time, payload); err != nil {
+		if _, err := h.api.CreateReminder(ctx, dec.Slots.Time, payload, ""); err != nil {
 			log.Printf("voice: create reminder: %v", err)
 			return "не получилось поставить напоминание."
 		}