feat: Add RunnerPool for shared queue polling

[MohamedBassem]

Summary

• Add RunnerPool for polling multiple queues through a shared database loop.
• Preserve per-queue handlers, validators, timeouts, and concurrency limits.
• Add atomic cross-queue dequeue support and public exports.
• Document RunnerPool usage and add coverage for pooling behavior.

Testing

• Added Vitest coverage for independent queue handling, concurrency limits, stopping idle polls, and database validation.
• Not run.