fix(auth): fix inviter name access in sendInvitationEmail

Access inviter.user.name instead of inviter.name to match Better Auth's
organization plugin type structure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/mana-core-auth/src/auth/better-auth.config.ts

@@ -139,7 +139,7 @@ export function createBetterAuth(databaseUrl: string) {
 					await sendInvitationEmail(
 						email,
 						organization.name,
-						inviter?.name || 'Ein Teammitglied',
+						inviter?.user?.name || 'Ein Teammitglied',
 						inviteUrl
 					);
 				},