fix(6244): the instance keeps its own SECRET_KEY in /data, and docker-compose.yml is the compose users deploy from

With no SECRET_KEY set, Grimoire fell back to a fixed placeholder. The
quickstart compose substituted change-me-in-production, so the README's
own update step (pull, up -d) recreated the container on that placeholder,
and every credential sealed under the key the user started with stopped
opening.

- config: with neither SECRET_KEY nor SECRET_KEY_FILE, the first start
  writes a random key to DATA_DIR/secret_key (mode 0600, published with a
  hard link so concurrent starts agree), and later starts read it back.
  An explicit key wins and leaves the file alone. A missing
  SECRET_KEY_FILE or an empty key file stops the start instead of
  switching keys. With no data directory (a run outside the container)
  the process gets a temporary key and says so.
- app: warns on a public placeholder key, on a temporary key, and when
  /data is not a mounted volume.
- Dockerfile creates /data.
- docker-compose.yml is now the deploy compose (published image, both
  volumes, nothing to set). The build-from-source compose is
  docker-compose.dev.yml, and docker-compose.quickstart.yml is gone. The
  Swarm stack file gains the /data volume and keeps its explicit key,
  because a Swarm can move the app away from a node-local volume.
- README: a Deploy section with the compose inline, SECRET_KEY optional,
  "keep both volumes", and updates that keep the key. A test holds the
  README's copy equal to the file.
- Installs that ran on the change-me-in-production placeholder get a key
  of their own on the new compose; Settings then asks once for saved
  forge tokens and the backup secret.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -56,5 +56,9 @@ ENV APP_BUILD_KEY=$BUILD_KEY
 ENV APP_CHANNEL=$BUILD_CHANNEL
 ENV APP_COMMIT=$BUILD_COMMIT
 
+# The instance's own SECRET_KEY, the model cache and diagnostics live here.
+# Mount a volume on it so they outlive the container.
+RUN mkdir -p /data
+
 EXPOSE 5000
 CMD ["sh", "-c", "alembic upgrade head && hypercorn 'grimoire.app:create_app()' --bind 0.0.0.0:5000 --keep-alive 600"]