feat(atomize): serve Subdivide via Anypreta Issue Atomizer

Add an optional remote atomization backend (the Anypreta "Issue Atomizer"
API) for the Subdivide flow, selected by ATOMIZER_REMOTE_BASE_URL /
ATOMIZER_REMOTE_API_KEY. When set, POST /v1/atomize is served by the remote
/v1/atomize-issue endpoint; its two-level issue→features→tasks model is mapped
onto our one-level task→children model by treating each Feature as a child and
its `estimation` as the effort coefficient (renormalized to sum to 1, even
split as a reported fallback when estimations are absent).

Extend and Summarize have no counterpart in the remote API and always stay on
ATOMIZER_BASE_URL, so the §5.1 service must keep running; readiness now probes
both backends. The mandatory `system` object is derived from the customer,
with a generic component tree (an empty tree makes the service return zero
features).

extsvc also learns the remote's flat error envelope ({error_code,message} and
FastAPI {detail}) and never swallows an unrecognized 4xx body.

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

.env.example

@@ -24,6 +24,12 @@ ADMIN_INITIAL_PASSWORD=change-me-now
 ATOMIZER_BASE_URL=http://atomizer-mock:8090
 ATOMIZER_TOKEN=change-me-atomizer
 ATOMIZER_TIMEOUT_SEC=120
+# Optional: serve Subdivide from the Anypreta Issue Atomizer instead of the
+# §5.1 service above. Both vars are required to enable it. Extend and Summarize
+# have no counterpart in that API and always stay on ATOMIZER_BASE_URL, so the
+# service above must keep running either way.
+ATOMIZER_REMOTE_BASE_URL=
+ATOMIZER_REMOTE_API_KEY=
 WORK_PERFORMER_BASE_URL=http://work-performer:8091
 WORK_PERFORMER_TOKEN=change-me-performer
 # Job submission/polling only; jobs themselves are async.