[New Feature] Public /discover page for opt-in tenant catalog

What: Public catalog at /discover on the marketing site listing every Mobieus tenant that opted in. Filterable by category, member count, free/paid. Each tenant gets a 1-line pitch + member-count badge + link to join. Opt-in only — non-opted tenants are invisible.

Who benefits: Tenant operators (free customer-acquisition channel that drives sign-ups to their community); us (sticky tenants stay longer).

Why: Skool's discovery page is their #1 acquisition channel. It is the single highest-ROI quick win in this plan. Free for tenants that opt in; zero effect on those who do not.

Dependencies: None. The marketing site already has the IA to add a new top-level surface.

Recommended order: Sprint 1 (week 1-2). Ship first.

Complexity: 1 week.

Priority: critical (highest-impact quick win)