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>

internal/models/reminder_log.go

@@ -0,0 +1,91 @@
+package models
+
+import (
+	"fmt"
+	"time"
+)
+
+// ReminderStage represents the stage of a task reminder
+type ReminderStage string
+
+const (
+	// Pre-due date reminders (days before)
+	ReminderStage30Days ReminderStage = "reminder_30d"
+	ReminderStage14Days ReminderStage = "reminder_14d"
+	ReminderStage7Days  ReminderStage = "reminder_7d"
+	ReminderStage3Days  ReminderStage = "reminder_3d"
+	ReminderStage1Day   ReminderStage = "reminder_1d"
+	ReminderStageDayOf  ReminderStage = "day_of"
+
+	// Overdue reminders (days after)
+	ReminderStageOverdue1  ReminderStage = "overdue_1"
+	ReminderStageOverdue2  ReminderStage = "overdue_2"
+	ReminderStageOverdue3  ReminderStage = "overdue_3"
+	ReminderStageOverdue4  ReminderStage = "overdue_4"
+	ReminderStageOverdue7  ReminderStage = "overdue_7"
+	ReminderStageOverdue10 ReminderStage = "overdue_10"
+	ReminderStageOverdue13 ReminderStage = "overdue_13"
+)
+
+// GetReminderStageForDaysBefore returns the reminder stage for days before due date
+func GetReminderStageForDaysBefore(days int) ReminderStage {
+	switch days {
+	case 30:
+		return ReminderStage30Days
+	case 14:
+		return ReminderStage14Days
+	case 7:
+		return ReminderStage7Days
+	case 3:
+		return ReminderStage3Days
+	case 1:
+		return ReminderStage1Day
+	case 0:
+		return ReminderStageDayOf
+	default:
+		// For any other number of days, create a custom stage
+		return ReminderStage(fmt.Sprintf("reminder_%dd", days))
+	}
+}
+
+// GetReminderStageForDaysOverdue returns the reminder stage for days overdue
+func GetReminderStageForDaysOverdue(days int) ReminderStage {
+	switch days {
+	case 1:
+		return ReminderStageOverdue1
+	case 2:
+		return ReminderStageOverdue2
+	case 3:
+		return ReminderStageOverdue3
+	case 4:
+		return ReminderStageOverdue4
+	case 7:
+		return ReminderStageOverdue7
+	case 10:
+		return ReminderStageOverdue10
+	case 13:
+		return ReminderStageOverdue13
+	default:
+		// For any other overdue day, format as overdue_N
+		return ReminderStage(fmt.Sprintf("overdue_%d", days))
+	}
+}
+
+// TaskReminderLog tracks which reminders have been sent to prevent duplicates
+type TaskReminderLog struct {
+	ID             uint          `gorm:"primaryKey" json:"id"`
+	TaskID         uint          `gorm:"column:task_id;not null;index:idx_reminderlog_task_user_date" json:"task_id"`
+	Task           *Task         `gorm:"foreignKey:TaskID" json:"-"`
+	UserID         uint          `gorm:"column:user_id;not null;index:idx_reminderlog_task_user_date" json:"user_id"`
+	User           *User         `gorm:"foreignKey:UserID" json:"-"`
+	DueDate        time.Time     `gorm:"column:due_date;type:date;not null;index:idx_reminderlog_task_user_date" json:"due_date"`
+	ReminderStage  ReminderStage `gorm:"column:reminder_stage;size:20;not null" json:"reminder_stage"`
+	SentAt         time.Time     `gorm:"column:sent_at;default:CURRENT_TIMESTAMP;index:idx_reminderlog_sent_at" json:"sent_at"`
+	NotificationID *uint         `gorm:"column:notification_id" json:"notification_id"`
+	Notification   *Notification `gorm:"foreignKey:NotificationID" json:"-"`
+}
+
+// TableName returns the table name for GORM
+func (TaskReminderLog) TableName() string {
+	return "task_reminderlog"
+}