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/ipc_test.go

@@ -311,7 +311,7 @@ func TestClient_E2E(t *testing.T) {
 	}
 
 	// reminder lifecycle: create → mark fired → re-mark ⇒ ErrReminderState.
-	rid, err := cli.CreateReminder(ctx, now.Add(time.Hour), `{"text":"wake me 7"}`)
+	rid, err := cli.CreateReminder(ctx, now.Add(time.Hour), `{"text":"wake me 7"}`, "")
 	if err != nil {
 		t.Fatalf("CreateReminder: %v", err)
 	}