Add AsyncWorker, async dispatch and serverless

[nikhilvdev]

Implements Phase 6: a background AsyncWorker and non-blocking Logger dispatch. Logger gains async_dispatch, max_queue_size and backpressure (drop_oldest/drop_newest/block), plus flush()/flush_async()/close(). Child loggers share the parent's worker. Adds serverless helpers (with_lambda / with_cloud_function / with_azure_function) that flush loggers before handler return. Transport.flush() hook introduced. Config loading maps new async keys. Documentation (README/CHANGELOG) updated and comprehensive tests added for worker, async logging and serverless behavior.

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope