docs: frame the Python client within the Chronicle client family and extend package keywords

[woksin]

What changed

• README.md — the intro now describes Chronicle as the open-source (MIT) event-sourcing database and processing runtime — an event store for event-sourced and CQRS applications — and adds one framing sentence placing Python alongside the existing .NET, TypeScript, Kotlin/Java, and Elixir Chronicle clients, with links to each client repository. The pre-alpha / no-commitment disclaimer is unchanged.
• pyproject.toml — extends the package keywords with cqrs, event-store, eventsourcing, ddd, async (and cratis) so the package is findable under common event-sourcing search terms once it is eventually published. The Pre-Alpha development-status classifier is kept.

Why

Better discoverability and accurate breadth: people searching for event sourcing, event store, or CQRS in Python should find this repository and understand where it fits in the Chronicle client family. No shipped claim is made — the client remains pre-alpha, unpublished, and coming soon.

No code changes; docs and package metadata only.