- Fix NULL handling for FK checks in stratplays.py: use x.field_id instead
of x.field to avoid triggering FK lookups on potentially missing rows
- Cast boolean is_start to integer for SUM() - PostgreSQL cannot sum booleans
- Add missing GROUP BY clause to Decision aggregate query
- Add Case import for boolean-to-integer casting
- Update migration script with boolean/datetime column mappings
- Exclude legacy battingstat/pitchingstat tables from migration
- Add comprehensive POSTGRES_MIGRATION_GUIDE.md documentation
Tested: /plays/batting and /plays/pitching endpoints work with group_by=player
