fix: queue concurrent streaming requests to avoid ~60s delay

[BP602]

Summary

• serialize Claude Agent SDK requests with p-queue to avoid the ~60s delay when concurrent streaming requests are made
• add p-queue dependency and route all /messages requests through the queue
• keeps streaming behavior intact while ensuring sequential execution

Testing

• concurrent curl streaming requests (2 parallel requests) => ~12s total vs ~67s previously

[BP602]

Fixes #11