Split the worker into a distribution of its own

A cluster or GPU host installs `pip install fluksio-worker` and gets the
agent and the runner, not psycopg, numpy and the MCP SDK. The engine
depends on it as a workspace member, so the file it launches node code
with is the same file a remote worker runs — which is what keeps a node
unable to tell the difference. Copying the two files by hand still works.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -33,6 +33,11 @@ COPY ./backend/pyproject.toml ./backend/alembic.ini /app/backend/
 
 COPY ./backend/fluksio /app/backend/fluksio
 
+# The worker is a workspace member of its own, so the engine's sync needs it
+# present to resolve the dependency on it. It is also what the engine runs
+# node code with.
+COPY ./worker /app/worker
+
 # Sync the project
 # Ref: https://docs.astral.sh/uv/guides/integration/docker/#intermediate-layers
 RUN --mount=type=cache,target=/root/.cache/uv \