docTR called .cuda() when torch happened to see a card and otherwise simply
stayed on the CPU, so a GPU-less install read every page many times slower
with nothing in the response to say why — the result looked normal, it just
arrived minutes late. Nothing else in coderai quietly relocates GPU work, and
paddle refuses outright in exactly this situation; docTR was the odd one out.

`doctr_use_gpu` is now a requirement: no CUDA device means a 503 that names
the setting. Running docTR on the CPU is still fully supported and unchanged —
it just has to be asked for, with doctr_use_gpu = false.

The test drives the real load() with docTR and torch stubbed, rather than
re-implementing the placement rules beside it; reintroducing the fallback
makes it fail, which is the only reason to trust it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PxYHWbCAjjjD7A1rhFKtaW