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>

ocr-service/README.md

@@ -32,6 +32,10 @@ Python FastAPI microservice that performs the actual handwritten text recognitio
 | `ALLOWED_PDF_HOSTS` | `minio,localhost,127.0.0.1` | YES | — | SSRF protection — comma-separated allowed PDF source hosts. Never set to `*`. |
 | `KRAKEN_MODEL_PATH` | `/app/models/` | — | — | Directory where Kraken HTR models are stored (populated by `download-kraken-models.sh`) |
 | `BLLA_MODEL_PATH` | `/app/models/blla.mlmodel` | — | — | Kraken baseline layout analysis model. Auto-downloaded via `ensure_blla_model.py` on startup if missing. |
+| `HF_HOME` | `/app/cache` | — | — | HuggingFace model cache root. Keeps model downloads on the persistent cache volume. |
+| `XDG_CACHE_HOME` | `/app/cache` | — | — | XDG cache root (used by some Surya components alongside `HF_HOME`). |
+| `TORCH_HOME` | `/app/models/torch` | — | — | PyTorch model cache. Kept on the persistent models volume. |
+| `TMPDIR` | `/app/cache/.tmp` | — | — | Download-staging directory for GB-scale Surya model files. Must point to a disk-backed path, not the 512 MB `/tmp` tmpfs — see ADR-021. |
 
 ## Key files