List the leagues visible to the tenant. Tenants entitled to the CRYPTO league (a
sportsstack_feed entitlement for CRYPTO) additionally see the synthetic CRYPTO
league, whose sport_id is the crypto sport, exactly once across all pages (it is the
last element of the list, so a full last page pushes it to the next page, and
x-total-count includes it). Filter with sport_id set to the crypto sport to list it
alone.

Each item carries futures_count, the number of tenant-visible future
markets in that league (the same set GET /futures?league_id=<league>
pages through); the synthetic CRYPTO league has no futures and reports 0.
Discovery flow for outrights and season markets:
GET /sports -> GET /leagues (skip leagues with futures_count 0) ->
GET /futures?league_id=<abbr>&season_year=<year>&aggregator=<venue>.