build(ocr): set ENV TMPDIR=/app/cache/.tmp so docker run uses SSD staging

Without this, running the image outside compose loses the TMPDIR redirect
and Surya model downloads fall back to the 512 MB /tmp tmpfs (ENOSPC).
See issue #614, ADR-021.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/Dockerfile

@@ -32,6 +32,7 @@ ENV HOME=/home/ocr
 ENV HF_HOME=/app/cache
 ENV XDG_CACHE_HOME=/app/cache
 ENV TORCH_HOME=/app/models/torch
+ENV TMPDIR=/app/cache/.tmp
 
 USER ocr