Replace old notification handlers with smart reminder system

- Add TaskReminderLog to AutoMigrate (table was missing)
- Remove HandleTaskReminder and HandleOverdueReminder registrations
- Register HandleSmartReminder which uses frequency-aware scheduling
  and tracks sent reminders to prevent duplicates

The old handlers sent ALL overdue tasks daily regardless of age.
Smart reminder tapers off (daily for 3 days, then every 3 days, stops at 14).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/database/database.go

@@ -144,6 +144,7 @@ func Migrate() error {
 		&models.NotificationPreference{},
 		&models.APNSDevice{},
 		&models.GCMDevice{},
+		&models.TaskReminderLog{}, // Smart reminder tracking
 
 		// Subscription tables
 		&models.SubscriptionSettings{},