fix: correct migration chain for league settings columns (2.5.1)

Migration 0003_league_settings was chained to 0002 but the DB was already
at 0003_site_admin (an applied migration with no version file). Added a stub
for 0003_site_admin to satisfy the chain and updated the down_revision on
0003_league_settings accordingly. Columns eligible_min_sessions and
break_even_cents now apply cleanly on top of the existing schema.

migrations/versions/0003_league_settings.py

@@ -11,7 +11,7 @@ import sqlalchemy as sa
 
 
 revision = "0003_league_settings"
-down_revision = "0002_league_public_key"
+down_revision = "0003_site_admin"
 branch_labels = None
 depends_on = None