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-db/src/repo/topology_runs.rs

@@ -187,6 +187,37 @@ pub async fn touch(pool: &PgPool, id: Uuid) -> Result<(), DbError> {
     Ok(())
 }
 
+/// If this run belongs to a research topic AND no siblings of that topic
+/// are still queued or running, transition the topic `processing → reviewing`.
+/// Guarded by `status = 'processing'` so a repeat call (e.g. a retry) is a
+/// no-op; a topic already reviewing/publishing/published stays put.
+/// Returns `true` when the topic was transitioned.
+pub async fn notify_run_completed(pool: &PgPool, id: Uuid) -> Result<bool, DbError> {
+    // One statement: subquery locates the topic id, subquery counts siblings
+    // still in flight (excluding *this* run — it's about to be flipped to
+    // completed/failed by the caller, but ordering isn't guaranteed here).
+    let row = sqlx::query!(
+        "UPDATE research_topics t
+            SET status = 'reviewing', updated_at = now()
+          WHERE t.id = (
+                    SELECT research_topic_id FROM topology_runs
+                     WHERE id = $1 AND research_topic_id IS NOT NULL
+                )
+            AND t.status = 'processing'
+            AND NOT EXISTS (
+                    SELECT 1 FROM topology_runs
+                     WHERE research_topic_id = t.id
+                       AND id <> $1
+                       AND status IN ('queued', 'running')
+                )
+          RETURNING t.id",
+        id,
+    )
+    .fetch_optional(pool)
+    .await?;
+    Ok(row.is_some())
+}
+
 /// Mark a job completed and store its final result blob.
 pub async fn complete(pool: &PgPool, id: Uuid, result: &Value) -> Result<(), DbError> {
     sqlx::query!(