Adds a sports_season_tournaments junction table and bidirectional
link/unlink UI on both the sports season and tournament admin pages.
- New junction table with unique index on (sports_season_id, tournament_id)
and a separate index on tournament_id for reverse lookups
- New model with link/unlink/query functions and cross-sport validation
- Migration includes backfill from existing scoring_events tournament links
- Admin sports season page: Tournaments card with add/remove UI
- Admin tournament page: Linked Sports Seasons card with add/remove UI
- Inline success/error feedback, empty states, aria-labels on remove buttons
- cloneSportsSeason now copies tournament links
- Fixes darts simulator test timeout (15s for 128-player pre-bracket sim)
