research sidebar: delete-with-confirm per topic

Mirror the row-level delete affordance the loops sidebar already
has. Loops was wired earlier; research had a bare title-only card
with no way to remove a stale topic.

- cm-db: research_topics::delete cascades via existing FK rules
  (research_topic_agents, research_publish_approvals, and the new
  research_outcomes all CASCADE on topic_id; topology_runs's
  research_topic_id back-ref is SET NULL so historical runs stay).
- cm-api: DELETE /api/research/{id} → 204. Idempotent.
- Frontend: deleteTopic helper. ResearchList row is now a card
  with the existing title/status/outcome header plus a trash icon
  that flips the card into an inline "Delete topic + all outcomes?"
  confirm strip. Confirm → red Delete / gray Cancel. If the
  deleted topic was selected, selection clears; local counter
  bumps the list refetch without waiting on a parent.

crates/cm-db/src/repo/research_topics.rs

@@ -98,6 +98,21 @@ pub async fn get(
     Ok(row)
 }
 
+/// Hard-delete a topic and cascade every dependent row. FK cascades on
+/// research_topic_agents, research_publish_approvals, and research_outcomes
+/// clean themselves up; topology_runs.research_topic_id is SET NULL so
+/// historical runs survive with the back-ref cleared.
+pub async fn delete(pool: &PgPool, id: Uuid, workspace_id: Uuid) -> Result<(), DbError> {
+    sqlx::query!(
+        "DELETE FROM research_topics WHERE id = $1 AND workspace_id = $2",
+        id,
+        workspace_id,
+    )
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
 /// Non-status fields; the state machine transitions are their own endpoints.
 pub async fn update_fields(
     pool: &PgPool,