Add Postgres integration tests to CI

[kitplummer]

Summary

tests/postgres_tests.rs (13 tests) exists but is never run in CI — it gracefully skips when Postgres is unavailable.

Add a Postgres Docker service to CI:

postgres-integration:
  services:
    postgres:
      image: postgres:16
      env:
        POSTGRES_PASSWORD: test
      ports: ["5432:5432"]
  env:
    POSTGRES_URL: postgres://postgres:test@localhost/peat_gateway_test
  steps:
    - run: cargo test --features postgres --test postgres_tests