feat(matchmaking): allow two-person matching

[Ltcc0]

Summary

• allow the Story Table activity and matching contracts to form rooms with two users
• add a Supabase migration lowering database constraints from three to two
• add matcher and migration regression coverage for a two-person round
• update the implementation specification

Validation

• pnpm --filter @tomeet/matchmaking test (5 passed)
• pnpm --filter @tomeet/data test (41 passed)
• pnpm --filter @tomeet/api test -- src/app.test.ts (21 passed)
• affected package typechecks passed
• targeted ESLint passed