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/api.go

@@ -51,13 +51,15 @@ type Note struct {
 	Score  float64   `json:"score"`
 }
 
-// Reminder — user-stated future intent; fires once.
+// Reminder — user-stated future intent; fires once or recurring (if cron set).
 type Reminder struct {
-	ID        int64     `json:"id"`
-	CreatedTs time.Time `json:"created_ts"`
-	FireTs    time.Time `json:"fire_ts"`
-	Payload   string    `json:"payload"`
-	Status    string    `json:"status"` // pending|fired|cancelled
+	ID         int64     `json:"id"`
+	CreatedTs  time.Time `json:"created_ts"`
+	FireTs     time.Time `json:"fire_ts"`
+	NextFireTs time.Time `json:"next_fire_ts"`
+	Payload    string    `json:"payload"`
+	Status     string    `json:"status"` // pending|fired|cancelled
+	Cron       string    `json:"cron"`
 }
 
 // Presence — the read the phraser / delivery modules need to decide channel
@@ -137,6 +139,7 @@ type queryNotesReq struct {
 type createReminderReq struct {
 	Fire    time.Time `json:"fire"`
 	Payload string    `json:"payload"`
+	Cron    string    `json:"cron"`
 }
 type recordNudgeReq struct {
 	Rule    string    `json:"rule"`
@@ -212,7 +215,7 @@ type CoreAPI interface {
 	LatestFactBySource(ctx context.Context, key, source string) (Fact, error)
 	Since(ctx context.Context, key string, now time.Time) (time.Duration, error)
 	Presence(ctx context.Context) (Presence, error)
-	CreateReminder(ctx context.Context, fire time.Time, payload string) (int64, error)
+	CreateReminder(ctx context.Context, fire time.Time, payload, cron string) (int64, error)
 	MarkReminder(ctx context.Context, id int64, status string) error
 	RecordNudge(ctx context.Context, rule, channel, message string, ts time.Time) (int64, error)
 	ResolveNudge(ctx context.Context, id int64, outcome string, ts time.Time) error