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

go.sum

@@ -12,6 +12,8 @@ github.com/ncruces/go-strftime v0.1.9 h1:bY0MQC28UADQmHmaF5dgpLmImcShSi2kHU9XLdh
 github.com/ncruces/go-strftime v0.1.9/go.mod h1:Fwc5htZGVVkseilnfgOVb9mKy6w1naJmn9CehxcKcls=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
+github.com/robfig/cron/v3 v3.0.1 h1:WdRxkvbJztn8LMz/QEvLN5sBU+xKpSqwwUO1Pjr4qDs=
+github.com/robfig/cron/v3 v3.0.1/go.mod h1:eQICP3HwyT7UooqI/z+Ov+PtYAWygg1TEWWzGIFLtro=
 github.com/yalue/onnxruntime_go v1.31.0 h1:1ln4YW1SFOFfGJZXe3jNOb2JUSt+l2pEneZfV8HdtFA=
 github.com/yalue/onnxruntime_go v1.31.0/go.mod h1:b4X26A8pekNb1ACJ58wAXgNKeUCGEAQ9dmACut9Sm/4=
 golang.org/x/mod v0.16.0 h1:QX4fJ0Rr5cPQCF7O9lh9Se4pmwfwskqZfq5moyldzic=