MSPs onboarding customers as separate orgs had no way to create one with
AUTO_CREATE_ORG_ON_SIGNUP=false (the only path was signup auto-org). Add a
platform-admin 'Create organization' action.
POST /api/admin/orgs (requirePlatformAdmin) creates the org + its first 'Default'
workspace. organizations.owner_user_id is NOT NULL, so an org can't be ownerless;
the creating admin becomes org_owner + workspace_admin (mirrors the signup
bootstrap in routes/auth.js) - which also surfaces the org in their switcher.
Customer users are then added via the existing Add User / manage-memberships flow.
UI: 'Create organization' button + single-field modal in the Admin area (gated).
Tests: create (201 + memberships + audit), empty-name 400, non-admin/operator 403.
