Fall back to `con.exchange` in IB ledger fill loop

Use `con.primaryExchange or con.exchange` so
`pexch` is populated even when `primaryExchange`
is empty (e.g. for certain combo/forex fills).

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/brokers/ib/ledger.py

@@ -501,7 +501,7 @@ async def update_ledger_from_api_trades(
     for fill in fills:
         con: Contract = fill.contract
         conid: str = con.conId
-        pexch: str | None = con.primaryExchange
+        pexch: str|None = con.primaryExchange or con.exchange
 
         if not pexch:
             cons = await client.get_con(conid=conid)