feat(infra): parameterize OCR mem_limit via OCR_MEM_LIMIT

Hardcoded `mem_limit: 12g` only works on CX42+ (16 GB) hosts; a CX32 (8
GB) cannot honour it. Make both mem_limit and memswap_limit driven by
the OCR_MEM_LIMIT env var, defaulting to 12g so prod deploys on a CX42
keep current behaviour. Operators on smaller hosts override to 6g.

Verified compose interpolation produces 12 GiB by default and 6 GiB when
OCR_MEM_LIMIT=6g.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -121,8 +121,10 @@ services:
       - "8000"
     # Surya OCR loads ~5GB of transformer models at startup; first request
     # triggers a further ~1GB Kraken model download into ocr-cache.
-    mem_limit: 12g
-    memswap_limit: 12g
+    # CX42+ (16 GB RAM) honours the default. On a CX32 (8 GB) override with
+    # OCR_MEM_LIMIT=6g (slower first-request, fits the host).
+    mem_limit: ${OCR_MEM_LIMIT:-12g}
+    memswap_limit: ${OCR_MEM_LIMIT:-12g}
     volumes:
       - ocr-models:/app/models
       - ocr-cache:/root/.cache