Public: resolve a school slug to its branding + active join code.

Powers the /join/<slug> landing page so incoming students get a link with the school baked in (no code to type). Returns the org-stamped invite code to stash client-side; redemption (which creates the OrganizationMember and grants premium) still happens through the normal redeem path after signup. Invite codes are meant to be distributed in the school's welcome email, so surfacing one for a known slug is intentional, not a leak.