Store cook log DB in writable data dir, not read-only recipes mount

The cook log SQLite DB was created at recipeDir/cook-log.db. In the Docker
deployment /recipes is mounted read-only, so SQLite could never create the
file there — the cook log has never worked in the container. The writable
/data volume was mounted but unused.

- initDb now creates the DB's parent directory if missing, so startup
  initialization is self-contained and idempotent.
- app takes a dataDir and places cook-log.db there.
- Add optional --data-dir flag (defaults to the recipe dir, preserving
  local-dev behavior); Docker CMD passes --data-dir /data.

Verified end-to-end as root against a read-only recipe dir and a
non-existent data dir: the dir and DB are created and cook-log POST/GET
round-trips.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -5,5 +5,7 @@ services:
     ports:
       - "127.0.0.1:8080:8080"
     volumes:
+      # Cooklang source of truth, managed externally — mounted read-only.
       - /srv/roux/recipes:/recipes:ro
+      # Writable app state (the cook log SQLite DB); see --data-dir in the Dockerfile CMD.
       - /srv/roux/data:/data