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)

db_models.py

@@ -93,6 +93,8 @@ class League(TimestampMixin, db.Model):
         index=True,
     )
     archived_at = db.Column(db.DateTime(timezone=True), nullable=True)
+    eligible_min_sessions = db.Column(db.Integer, nullable=False, default=3)
+    break_even_cents = db.Column(db.Integer, nullable=False, default=100)
 
     __table_args__ = (
         db.CheckConstraint(