Split unit vs integration tests; drive via Just commands

[MatthewCane]

Rationale

Faster local feedback for unit tests without Docker; clear commands to run suites explicitly.

Files

• tests/conftest.py, tests under tests/
• pyproject.toml (pytest config)
• Justfile

Tasks

• Introduce @pytest.mark.integration for tests requiring Docker
• Replace the autouse Docker Compose session fixture with an integration-only fixture used by tests marked integration
• Update tests to use the integration fixture or add the @pytest.mark.integration marker
• Update pytest config to stop auto-marking all tests
• Add Just recipes:
  • pytest-unit: uv run pytest -v -m "not integration"
  • pytest-integration: uv run pytest -v -m integration
  • pytest-all: run both: just pytest-unit && just pytest-integration
• Optionally keep pytest as an alias to pytest-all

Acceptance criteria

• just pytest-unit passes locally without Docker
• just pytest-integration runs only integration tests (with Docker) and passes
• just pytest-all runs unit then integration suites and passes
• just --list shows the three new recipes with clear descriptions