fix: synchronous endpoint + no semaphore, let Cloud Run scale like Modal (#282)

The fire-and-forget + semaphore design caused all jobs to queue on one
instance. Cloud Run couldn't see background threads as "busy" so it
never scaled to new instances.

Fix: make endpoint synchronous (await executor) with concurrency=1.
Cloud Run sees each request as active during processing and scales to
new GPU instances for concurrent jobs — matching Modal's .spawn().
Increase client POST timeout to 1800s to match.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>