Adds a design document for supporting admins who manage 2+ specific brands
(e.g., franchise / multi-brand tenant use cases).
Current model: admin_users.brand_id is a single UUID | null, and the
effectiveBrandId = brandId ?? adminUser.brand_id ?? null pattern silently
does the wrong thing for multi-brand admins. There is no central
validation that an admin is acting in a brand they have access to.
Proposed: admin_user_brands junction table (m:n), kept admin_users.brand_id
for backwards compat, a new multi_brand_admin role, a cookie-based active
brand, a centralized brand-scope helper, and a BrandSelector dropdown in
the admin header. ~30 server actions and ~10 page server components get
a mechanical one-line swap to use the new helper.
Follow-up migration 220_drop_legacy_brand_id.sql will drop the legacy
column after we verify nothing reads it. Out of scope for this spec.
See docs/superpowers/specs/2026-06-04-multi-brand-admin-design.md
tyler