fix(objectql): use valid scope value for project overlay rows

protocol.saveMetaItem was writing scope='project' for per-project
customizations, but the sys_metadata.scope enum is
['system', 'platform', 'user']. Every Studio overlay write
("Duplicate view", edit field, etc.) on a tenant environment
hit Field.select validation and returned 500
overlay_persistence_failed.

Project isolation is already provided by the project_id column
(part of the overlay-active unique index), so the scope label
is informational. Always write 'platform' — the value the schema
already defaults to.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: hotlong

packages/objectql/src/protocol.ts

@@ -1742,7 +1742,10 @@ export class ObjectStackProtocolImplementation implements ObjectStackProtocol {
                     type: request.type,
                     // `scope` is informational; `project_id` is the authoritative
                     // isolation key (see sys-metadata schema unique index).
-                    scope: this.projectId !== undefined ? 'project' : 'platform',
+                    // We always write 'platform' here — per-project overlays
+                    // are distinguished by `project_id`, not by a separate
+                    // scope value (the enum is ['system','platform','user']).
+                    scope: 'platform',
                     metadata: JSON.stringify(request.item),
                     state: 'active',
                     version: 1,