Add complete MLS stadium data following established sport module pattern:
- 30 MLS stadiums with capacity (soccer configuration) and year_opened
- MLS_TEAMS dict with all 30 teams
- get_mls_team_abbrev() function for team abbreviation lookup
- scrape_mls_stadiums_hardcoded() as primary source
- scrape_mls_stadiums_gavinr() as fallback source
- MLS_STADIUM_SOURCES configuration for fallback system
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
