refactor(domain): rename stadium_name/stadium_capacity to arena_name/arena_capacity

[NicoRuedaA]

Summary

• Renames all stadium_name and stadium_capacity fields to arena_name and arena_capacity
• Updates 54 files across Rust backend, TypeScript frontend, and JSON data files
• cargo check passes

Why

Part of the football to LoL migration effort. "Stadium" is football terminology; "Arena" is the standard term used in esports and League of Legends.

Scope

Layer	Files Changed
Rust domain	team.rs, definitions.rs, mod.rs, finances.rs
Rust DB	v001_initial_schema.sql, team_repo.rs, academy_team_persistence.rs
Rust commands	world.rs
Rust generator	world_io.rs
JSON data	default_teams.json, lec_world.json
TypeScript types	types.ts
TypeScript tests	30+ test files with updated fixtures

Checklist

• cargo check passes
• No remaining references to stadium_name or stadium_capacity in source code
• JSON data files updated
• Database schema updated

Closes #55