fix(events): add optional side field to OrderFilledEvent

OrderFilledEvent had no side field, forcing api/main.py's chart() to
backfill it from the sibling order_submitted row. Add an additive-only
`side: str | None = None` field (existing payloads without it still
parse) and populate it at the sole production emit site
(alpaca_paper/execution.py _finalize) from the ExecutionRequest already
in scope. The api/main.py backfill workaround is left in place as a
harmless fallback for pre-existing events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

alpaca_paper/execution.py

@@ -216,11 +216,13 @@ class AlpacaPaperExecutor:
                     order_id=request.order_id,
                     fill_price=fill_price or 0.0,
                     fill_quantity=filled_qty,
+                    side=request.side,
                     payload={
                         "symbol": request.symbol,
                         "order_id": request.order_id,
                         "fill_price": fill_price,
                         "fill_quantity": filled_qty,
+                        "side": request.side,
                     },
                 )
             )