refactor(domain): migrate PlayerSeasonStats to LoL terminology

[NicoRuedaA]

Summary

• Renamed goals field to kills in PlayerSeasonStats struct to use LoL terminology
• Updated all references across the codebase:
  • end_of_season.rs: Updated to use player.stats.kills when creating career entry
  • post_match.rs: Updated to add kills to player.stats.kills
  • round_summary.rs, news.rs, season_awards.rs: Updated top scorer logic
  • Tests in turn_tests.rs, end_of_season_tests.rs, player_repo.rs

Changes

• player.rs: Changed goals: u32 to kills: u32 in PlayerSeasonStats struct
• Updated all code that references stats.goals to use stats.kills

Testing

• cargo check passes

Refs: #52