Replace supabase-py stubs with working psycopg2 implementation using
Supabase pooler connection string. Implement full business logic in
repository (avg cost basis, cash accounting, trade recording, snapshots).
Add 12 unit tests + 4 integration tests (51 total portfolio tests pass).
Fix cash_pct bug in models.py, update docs for psycopg2 + pooler pattern.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
a17e5f3707