Add per-user notification time preferences

Allow users to customize when they receive notification reminders:
- Add task_due_soon_hour, task_overdue_hour, warranty_expiring_hour fields
- Store times in UTC, clients convert to/from local timezone
- Worker runs hourly, queries only users scheduled for that hour
- Early exit optimization when no users need notifications
- Admin UI displays custom notification times

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

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

internal/models/notification.go

@@ -23,6 +23,12 @@ type NotificationPreference struct {
 
 	// Email preferences
 	EmailTaskCompleted bool `gorm:"column:email_task_completed;default:true" json:"email_task_completed"`
+
+	// Custom notification times (nullable, stored as UTC hour 0-23)
+	// When nil, system defaults from config are used
+	TaskDueSoonHour      *int `gorm:"column:task_due_soon_hour" json:"task_due_soon_hour"`
+	TaskOverdueHour      *int `gorm:"column:task_overdue_hour" json:"task_overdue_hour"`
+	WarrantyExpiringHour *int `gorm:"column:warranty_expiring_hour" json:"warranty_expiring_hour"`
 }
 
 // TableName returns the table name for GORM