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/schema.sql

@@ -24,7 +24,7 @@ CREATE TABLE IF NOT EXISTS facts (
 CREATE INDEX IF NOT EXISTS idx_facts_key_ts ON facts (key, ts DESC);
 CREATE INDEX IF NOT EXISTS idx_facts_voids  ON facts (voids_id);
 
--- reminders — user intent, fires once.
+-- reminders — user intent, fires once or recurring (if cron set).
 -- relative→absolute happens at capture ("in 4h" → store now+4h, never the string).
 CREATE TABLE IF NOT EXISTS reminders (
   id          INTEGER PRIMARY KEY AUTOINCREMENT,