- Remove card_1..card_26 FK columns from Roster ORM model
- Add RosterSlot model with (roster, slot, card) and a unique index on (roster, slot)
- Activate get_cards() helper on Roster using the new junction table
- Register RosterSlot in create_tables for SQLite dev environments
- Add migrations/migrate_roster_junction_table.py to backfill existing data
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>