Backend: SpaceAccount Settings Change Matrix

SpaceAccount settings changes must stay cross-layer consistent.

Rationale:
- Settings shape drift across model, sync, serializer, and API layers causes silent data loss or invalid updates.

Scope:
- PR review policy for changes in SpaceAccount settings model/sync/API files.

Trigger A (model shape change):
- `SpaceAccountSettingsSyncModel` field add/remove/rename/type/default behavior changes.

Trigger B (patch contract change):
- Settings patch serializer/schema/API field contract changes.

Approval checks:
1) Trigger A includes corresponding sync schema/manager updates in the same PR, or explicit statement in PR explaining why no sync logic change is needed.
2) Trigger A includes migration changes when persisted DB schema is affected.
3) Trigger B keeps serializer and API patch schema aligned in the same PR.
4) Changed behavior includes test updates covering the modified settings contract.

Evidence:
- Diff alignment across model/sync/serializer/API files.
- Migration presence when DB shape changes.
- Tests assert changed settings behavior.

Decision:
- Return NOT APPROVED when Trigger A or Trigger B is met and cross-layer consistency evidence is missing.