research: auto-transition processing → reviewing on last run

Hooks the topology_worker's post-terminal path into a new
notify_run_completed repo helper that atomically transitions the topic
processing → reviewing when the completed run has research_topic_id set
AND no siblings for that topic are still queued or running. Guarded on
status='processing' so a retry, a re-fire, or a topic already past
processing are all no-ops. Best-effort at the worker; DB hiccups are
logged and never fail the run.

The manual /submit-review endpoint stays as an escape hatch for topics
that end up parked in processing with nothing to complete (updated the
doc comment).

crates/cm-api/src/routes/research.rs

@@ -244,9 +244,10 @@ pub async fn start_topic(
 }
 
 /// `POST /api/research/:id/submit-review` — flips status `processing → reviewing`.
-/// v1 is caller-driven: the UI hits this when the human is happy with the
-/// runs' output. A later commit hooks this from the orchestrator on the
-/// last topology_run's completion.
+/// Kept as a manual escape hatch: the orchestrator auto-transitions on the
+/// last topology_run's completion (see topology_worker's
+/// `notify_run_completed` hook), so callers only need this when there are
+/// no runs (e.g. a topic parked in `processing` with nothing in flight).
 pub async fn submit_review(
     State(state): State<AppState>,
     Authed(user): Authed,