feat(notify): convert email lists to NotifyEmailEntry struct with toggle support

- Change balance_notify_extra_emails and account_quota_notify_emails
  from []string to []NotifyEmailEntry{email, disabled, verified}
- Add per-email enable/disable toggle for both user and admin notifications
- Add PUT /user/notify-email/toggle API endpoint
- Fix critical bug: API key auth cache snapshot missing balance notify
  fields (Email, Username, BalanceNotifyEnabled, etc.), causing
  notifications to never fire on cached request paths
- Bump cache snapshot version 3→4 to invalidate stale entries
- Add SQL migration 104 to convert old format data
- Backward compatible: parseNotifyEmails auto-detects old/new format
- User balance notify: max 3 emails (primary + 2 extra)
- Admin quota notify: unlimited emails, each with toggle

backend/internal/repository/api_key_repo.go

@@ -3,7 +3,6 @@ package repository
 import (
 	"context"
 	"database/sql"
-	"encoding/json"
 	"fmt"
 	"strings"
 	"time"
@@ -667,12 +666,9 @@ func userEntityToService(u *dbent.User) *service.User {
 		CreatedAt:                  u.CreatedAt,
 		UpdatedAt:                  u.UpdatedAt,
 	}
-	// Parse extra emails JSON array
+	// Parse extra emails JSON (supports both old []string and new []NotifyEmailEntry format)
 	if u.BalanceNotifyExtraEmails != "" && u.BalanceNotifyExtraEmails != "[]" {
-		var emails []string
-		if err := json.Unmarshal([]byte(u.BalanceNotifyExtraEmails), &emails); err == nil {
-			out.BalanceNotifyExtraEmails = emails
-		}
+		out.BalanceNotifyExtraEmails = service.ParseNotifyEmails(u.BalanceNotifyExtraEmails)
 	}
 	return out
 }