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/handler/dto/notify_email_entry.go

@@ -0,0 +1,43 @@
+package dto
+
+import "github.com/Wei-Shaw/sub2api/internal/service"
+
+// NotifyEmailEntry represents a notification email with enable/disable and verification state.
+// Email="" is a placeholder for the "primary email" (user's registration email or first admin email).
+type NotifyEmailEntry struct {
+	Email    string `json:"email"`
+	Disabled bool   `json:"disabled"`
+	Verified bool   `json:"verified"`
+}
+
+// NotifyEmailEntriesFromService converts service entries to DTO entries.
+func NotifyEmailEntriesFromService(entries []service.NotifyEmailEntry) []NotifyEmailEntry {
+	if entries == nil {
+		return nil
+	}
+	result := make([]NotifyEmailEntry, len(entries))
+	for i, e := range entries {
+		result[i] = NotifyEmailEntry{
+			Email:    e.Email,
+			Disabled: e.Disabled,
+			Verified: e.Verified,
+		}
+	}
+	return result
+}
+
+// NotifyEmailEntriesToService converts DTO entries to service entries.
+func NotifyEmailEntriesToService(entries []NotifyEmailEntry) []service.NotifyEmailEntry {
+	if entries == nil {
+		return nil
+	}
+	result := make([]service.NotifyEmailEntry, len(entries))
+	for i, e := range entries {
+		result[i] = service.NotifyEmailEntry{
+			Email:    e.Email,
+			Disabled: e.Disabled,
+			Verified: e.Verified,
+		}
+	}
+	return result
+}