fix(coach): delete a conversation in one owner-scoped statement

A find-then-delete pair let two deletes of the same conversation both
find the row, and the slower one failed with a 500. A client that
re-sends a delete it could not confirm now gets 404 for the second
request, which it reads as already gone.

The integration test covers the thread, its messages and its document
links going with it, the document itself staying, the list no longer
carrying it, a foreign account getting 404, and the concurrent pair.

src/lib/ai/coach/persistence.ts

@@ -1094,21 +1094,24 @@ export async function renameConversation(
 }
 
 /**
- * Delete a conversation and every message under it. Returns false when
- * the conversation does not exist or is not owned by `userId` — the
- * route should map both to 404.
+ * Delete a conversation and every message under it (the foreign keys
+ * cascade). Returns false when the conversation does not exist or is not
+ * owned by `userId` — the route should map both to 404.
+ *
+ * One owner-scoped statement, like `renameConversation`: a client re-sends a
+ * delete it could not confirm (a reload inside the undo window), so two
+ * requests for the same row are expected. A find-then-delete pair let both
+ * find the row and the slower one fail with a 500; here one deletes it and
+ * the other reads 404.
  */
 export async function deleteConversation(
   userId: string,
   conversationId: string,
 ): Promise<boolean> {
-  const row = await prisma.coachConversation.findFirst({
+  const { count } = await prisma.coachConversation.deleteMany({
     where: { id: conversationId, userId },
-    select: { id: true },
   });
-  if (!row) return false;
-  await prisma.coachConversation.delete({ where: { id: row.id } });
-  return true;
+  return count === 1;
 }
 
 // ─── S7: coach-conversation attachments ─────────────────────────────────────