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)

migrations/versions/0003_league_settings.py

@@ -0,0 +1,32 @@
+"""add per-league eligible_min_sessions and break_even_cents
+
+Revision ID: 0003_league_settings
+Revises: 0002_league_public_key
+Create Date: 2026-06-26
+"""
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "0003_league_settings"
+down_revision = "0002_league_public_key"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("leagues") as batch_op:
+        batch_op.add_column(sa.Column(
+            "eligible_min_sessions", sa.Integer(), nullable=False, server_default="3"
+        ))
+        batch_op.add_column(sa.Column(
+            "break_even_cents", sa.Integer(), nullable=False, server_default="100"
+        ))
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("leagues") as batch_op:
+        batch_op.drop_column("break_even_cents")
+        batch_op.drop_column("eligible_min_sessions")