Return the thread ID properly down sync. (#14159)

A receipt's thread ID, if one exists, should be added to the
body of a receipt.

Author: clokep

changelog.d/14159.feature

@@ -0,0 +1 @@
+Support for thread-specific notifications & receipts ([MSC3771](https://github.com/matrix-org/matrix-spec-proposals/pull/3771) and [MSC3773](https://github.com/matrix-org/matrix-spec-proposals/pull/3773)).

synapse/storage/databases/main/receipts.py

@@ -418,6 +418,8 @@ def f(txn: LoggingTransaction) -> List[Dict[str, Any]]:
             receipt_type = event_entry.setdefault(row["receipt_type"], {})
 
             receipt_type[row["user_id"]] = db_to_json(row["data"])
+            if row["thread_id"]:
+                receipt_type[row["user_id"]]["thread_id"] = row["thread_id"]
 
         results = {
             room_id: [results[room_id]] if room_id in results else []