Prepare 1.3.1 release

[nickmoreton]

Summary

• bump the package and lockfile version to 1.3.1
• synchronize the runtime version identifier with the distribution metadata
• finalize the 1.3.1 changelog entry while preserving an empty Unreleased section

Why

This prepares the patch release containing the malformed honeypot timestamp fix currently on main. The runtime version constant was still reporting 1.2.1 after the prior release, so this release also brings it back in sync with the package metadata.

Impact

Malformed honeypot timestamps will be ignored instead of raising an exception, and consumers inspecting the runtime version will receive the published 1.3.1 version.

Verification

• uv lock --check
• package/runtime version consistency assertion
• uv run ruff check .
• uv run coverage run manage.py test (17 tests)
• uv build --sdist --wheel
• uvx twine check on both artifacts