Update usage settings

Assign workspace-wide usage policy. Assignment is currently unavailable until a writer to authoritative Billing configuration is integrated. The backend config platform.usageSettingsWritesEnabled is disabled by default; enabling it alone does not implement assignment, and an authorized request still returns 503 without changing configuration or persisting a second copy of policy. Requires admin.usage.settings:write and permission to edit workspace billing; a global administrator role is not required.