docs(ocr): document TMPDIR convention and add ADR-021

- ocr-service/README.md: add HF_HOME, XDG_CACHE_HOME, TORCH_HOME, TMPDIR rows
  to the environment variables table
- ocr-service/CLAUDE.md: LLM reminder — TMPDIR must stay on the cache volume
- docs/adr/021-tmpdir-persistent-volume-staging.md: records the decision,
  trade-offs, and rejected alternatives (Approach B / C) for issue #614
- ci.yml: add test_tmpdir.py to the OCR CI run (stdlib-only tests, no ML stack)

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

.gitea/workflows/ci.yml

@@ -148,7 +148,10 @@ jobs:
           path: frontend/test-results/screenshots/
 
   # ─── OCR Service Unit Tests ───────────────────────────────────────────────────
-  # Only spell_check.py, test_confidence.py, test_sender_registry.py — no ML stack required.
+  # Only stdlib/lightweight tests — no ML stack (PyTorch/Surya/Kraken) required.
+  # test_tmpdir.py covers the TMPDIR env var and entrypoint mkdir behaviour (ADR-021).
+  # test_tmpdir_is_inside_persistent_cache_volume is skipped in CI (TMPDIR not
+  # set to /app/cache here); it runs inside the deployed Docker container.
   ocr-tests:
     name: OCR Service Tests
     runs-on: ubuntu-latest
@@ -164,7 +167,7 @@ jobs:
         working-directory: ocr-service
 
       - name: Run OCR unit tests (no ML stack required)
-        run: python -m pytest test_spell_check.py test_confidence.py test_sender_registry.py -v
+        run: python -m pytest test_spell_check.py test_confidence.py test_sender_registry.py test_tmpdir.py -v
         working-directory: ocr-service
 
   # ─── Backend Unit & Slice Tests ───────────────────────────────────────────────