Add regression test for schema stale data and CI coverage enforcement

[jakebromberg]

The schema fix in #36 / PR #37 changed CREATE TABLE IF NOT EXISTS to DROP TABLE ... CASCADE + CREATE TABLE. A regression test is needed to prevent this from being reintroduced.

Additionally, CI should enforce a minimum coverage threshold to prevent coverage regressions.

Changes

1. Add test_schema_clears_stale_data_on_rerun: inserts data, re-runs schema, verifies tables are empty
2. Update CI test-postgres job to run all tests (unit + integration) with --cov-fail-under=80