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/store/migrations.go

@@ -17,7 +17,9 @@ import (
 //		`ALTER TABLE ...;`, // #1
 //	}
 var migrations = []string{
-	`ALTER TABLE tools ADD COLUMN scope TEXT NOT NULL DEFAULT 'homelab';`,
+	`ALTER TABLE tools ADD COLUMN scope TEXT NOT NULL DEFAULT 'homelab';`, // #1
+	`ALTER TABLE reminders ADD COLUMN cron TEXT;
+ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 }
 
 // migrate applies every migration with a number greater than the DB's current