gateway: ask a pod for the engine it can serve, in the request itself

The pod was launched with docTR as its default and still answered 'OCR engine
surya is not enabled' — because the request carried engine=surya, and the OCR
route honours the request's own field over the configured default. Setting the
pod's env changed the default; it did not change what the request asked for.

A pod target for OCR now rewrites the request's engine field on the way out,
to the engine the pod will actually serve (pod_ocr_engine mirrors the launch
env). Multipart is edited on the bytes with every other part untouched and
Content-Length corrected; verified through FastAPI's own parser with the real
probe body, file intact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY