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-api/src/routes/research.rs

@@ -324,6 +324,20 @@ pub async fn attach_agent(
     Ok(StatusCode::NO_CONTENT)
 }
 
+/// `DELETE /api/research/:id` — hard-delete a topic and cascade every
+/// dependent row. FK cascades on research_topic_agents,
+/// research_publish_approvals, and research_outcomes; topology_runs's
+/// research_topic_id back-ref is SET NULL so historical runs survive.
+/// Returns 204 whether the topic existed or not (idempotent).
+pub async fn delete_topic(
+    State(state): State<AppState>,
+    Authed(user): Authed,
+    Path(id): Path<Uuid>,
+) -> Result<StatusCode, ApiError> {
+    cm_db::repo::research_topics::delete(&state.pool, id, user.workspace_id.as_uuid()).await?;
+    Ok(StatusCode::NO_CONTENT)
+}
+
 pub async fn detach_agent(
     State(state): State<AppState>,
     Authed(user): Authed,