Add @pytest.mark.portal marker support (#37)

[ericof]

Summary

• Add @pytest.mark.portal marker that lets tests declare GenericSetup profiles, content, and roles inline — without overriding the portal fixture
• Add apply_profiles session-scoped fixture to apply GenericSetup profiles
• Add create_content session-scoped fixture to create content items as the site owner
• Add grant_roles session-scoped fixture to grant local roles to the test user
• Document all new fixtures and the marker in README.md

Closes #37

Example

@pytest.mark.portal(
    profiles=["my.addon:testing"],
    content=[{"type": "Document", "id": "doc1", "title": "Doc 1"}],
    roles=["Manager"],
)
def test_something(portal):
    assert "doc1" in portal

Housekeeping

• Bump Plone version to 6.1.4
• Add plone-stubs to test dependencies
• Update VS Code settings (remove obsolete flake8 config, add plone-stubs paths)

Test plan

• Existing tests without the marker pass unchanged (backwards-compatible)
• Tests with roles parameter grant roles to the test user
• Tests with content parameter create content in the portal
• Tests with profiles parameter apply GenericSetup profiles
• Combined marker parameters work together
• Marker setup doesn't leak between tests (integration layer rollback)