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.

Each item also carries market_count and market_counts_by_venue: how many
distinct OPEN markets (provider status active; suspended, closed, settled
and unknown statuses are not counted) the league's current events have, in
total and per venue (aggregator). Current events are the tenant-visible,
non-finished events that started within the last 36 hours or start within
the next 14 days. Only sources the tenant can read are counted (the visibility
GET /markets applies). market_count counts each market once;
market_counts_by_venue values can sum to more. An open market is not
necessarily priced right now. Values can be up to 60 seconds old. null
means the count could not be computed right now (never 0); the synthetic
CRYPTO league reports null.

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>.