Phase 6: Drop moves/passes tables, simplify schema to minimal index
Games table now only stores discovery index data: rkey, AT URI, players,
board_size, status, action_count, last_action_type, timestamps.
Removed: moves table, passes table, MoveRecord/PassRecord DB interfaces,
winner/score columns. Migration recreates table to drop old columns.
Updated server firehose placeholder to match new schema.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>