feat: per-league eligible sessions and break-even threshold settings (2.5.0)

Add two configurable leaderboard settings per league, replacing the global
constants with per-row DB columns:

- eligible_min_sessions (default 3): minimum sessions before a player ranks
  on the main leaderboard rather than showing as provisional
- break_even_cents (default 100 = $1.00): sessions within ±this of $0 net
  are classified as break-even rather than a win or loss

Changes:
- Migration 0003 adds both columns with server defaults for existing leagues
- League DB model gains eligible_min_sessions and break_even_cents columns
- net_result_bucket() and build_leaderboard() accept optional tolerance param
- summarize_player_runs() passes tolerance to net_result_bucket for streaks
- League leaderboard route reads values from the league row instead of config
- League settings form exposes both fields with validation (1-100 sessions,
  $0.00-$100.00 threshold)

config.py

@@ -9,7 +9,7 @@ DATA_PATH = BASE_DIR / "data" / "entries.csv"
 DEFAULT_DATABASE_URL = f"sqlite:///{BASE_DIR / 'data' / 'boker-dev.sqlite3'}"
 
 ELIGIBLE_MIN_SESSIONS = 3
-APP_VERSION = "2.4.0"
+APP_VERSION = "2.5.0"
 
 
 def load_local_env(env_path: Path) -> None: