* feat: add concurrency support to ProcessSupervisor
- Replace the single-slot mutex channel with a configurable semaphore to
allow multiple concurrent tasks.
- Add drain logic to ensure all active
tasks complete before process restarts.
* feat: add chromium-max-concurrency flag
- Add a --chromium-max-concurrency flag (1-6) to the Chromium module to
control how many conversions run in parallel.
- Update LibreOffice to pass maxConcurrency=1 as LibreOffice only supports
a single concurrent conversion.
* test: add integration tests for concurrent Chromium conversions
- Add concurrent request support to the integration test framework with
new step definitions for sending parallel requests and asserting on all
responses.
- Add a feature file for concurrent HTML to PDF conversions.
