fix(server): hide slug uniqueness constraint to prevent shared-link probe

Surfacing the Postgres unique-constraint name in the error response let
any authenticated user brute-force whether a custom slug was already in
use by another user's shared link, leaking the existence of other links.

server/src/services/shared-link.service.ts

@@ -110,7 +110,7 @@ export class SharedLinkService extends BaseService {
 
   private handleError(error: unknown): never {
     if ((error as PostgresError).constraint_name === 'shared_link_slug_uq') {
-      throw new BadRequestException('Shared link with this slug already exists');
+      throw new BadRequestException('Failed to save shared link');
     }
     throw error;
   }