Add Daily Digest notification preferences with custom time support

- Add daily_digest boolean and daily_digest_hour fields to NotificationPreference model
- Update HandleDailyDigest to check user preferences and custom notification times
- Change Daily Digest scheduler to run hourly (supports per-user custom times)
- Update notification service DTOs for new fields
- Add Daily Digest toggle and custom time to admin notification prefs page
- Fix notification handlers to only notify users at their designated hour

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

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

internal/models/notification.go

@@ -21,6 +21,9 @@ type NotificationPreference struct {
 	// Document notifications
 	WarrantyExpiring bool `gorm:"column:warranty_expiring;default:true" json:"warranty_expiring"`
 
+	// Summary notifications
+	DailyDigest bool `gorm:"column:daily_digest;default:true" json:"daily_digest"`
+
 	// Email preferences
 	EmailTaskCompleted bool `gorm:"column:email_task_completed;default:true" json:"email_task_completed"`
 
@@ -29,6 +32,7 @@ type NotificationPreference struct {
 	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"`
+	DailyDigestHour      *int `gorm:"column:daily_digest_hour" json:"daily_digest_hour"`
 }
 
 // TableName returns the table name for GORM