test: add PostgreSQL integration test against real database

[nao1215]

Roadmap: #1

Problem

There is no integration test that runs generated pog adapter code against an actual PostgreSQL database. The SQLite integration test (integration_test/sqlite_test.sh) exercises real CRUD operations against an in-memory SQLite database, but no equivalent exists for PostgreSQL.

Evidence

• integration_test/run.sh only runs sqlite_test.sh and compile_test.sh
• integration_test/compile_test.sh verifies that pog adapter code compiles, but never runs it
• test/codegen_test.gleam tests pog adapter code at the string-generation level only

Expected

Create integration_test/postgresql_test.sh that:

1. Starts or connects to a PostgreSQL instance (e.g., via Docker or CI service)
2. Generates native pog adapter code from a test schema and queries
3. Creates a temporary Gleam project with the generated code
4. Runs CRUD operations against the real database
5. Verifies correct results (insert, select one, select many, update, delete)
6. Tests PostgreSQL-specific features (RETURNING clause, ENUM types, UUID)

This can use a CI PostgreSQL service container in GitHub Actions.

Why This Matters

The pog adapter is a significant amount of generated code (~1000 lines) that has never been validated against a real PostgreSQL database. Bugs in query call construction, parameter encoding, or result decoding would go undetected.