Add smart notification reminder system with frequency-aware scheduling

Replaces one-size-fits-all "2 days before" reminders with intelligent
scheduling based on task frequency. Infrequent tasks (annual) get 30-day
advance notice while frequent tasks (weekly) only get day-of reminders.

Key features:
- Frequency-aware pre-reminders: annual (30d, 14d, 7d), quarterly (7d, 3d),
  monthly (3d), bi-weekly (1d), daily/weekly/once (day-of only)
- Overdue tapering: daily for 3 days, then every 3 days, stops after 14 days
- Reminder log table prevents duplicate notifications per due date/stage
- Admin endpoint displays notification schedules for all frequencies
- Comprehensive test suite (100 random tasks, 61 days each, 10 test functions)

New files:
- internal/notifications/reminder_config.go - Editable schedule configuration
- internal/notifications/reminder_schedule.go - Schedule lookup logic
- internal/notifications/reminder_schedule_test.go - Dynamic test suite
- internal/models/reminder_log.go - TaskReminderLog model
- internal/repositories/reminder_repo.go - Reminder log repository
- migrations/010_add_task_reminder_log.{up,down}.sql

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

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

migrations/010_add_task_reminder_log.down.sql

@@ -0,0 +1,5 @@
+-- Rollback: Smart Notification Reminder System
+
+DROP INDEX IF EXISTS idx_reminderlog_sent_at;
+DROP INDEX IF EXISTS idx_reminderlog_task_user_date;
+DROP TABLE IF EXISTS task_reminderlog;