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>
