"dashboard" documented on rebalancing run initiated_from

Broker API

The PortfolioRunInitiator schema description now spells out every value returned in initiated_from on rebalancing runs:

• system - runs Alpaca creates, such as scheduled rebalancing and subscription creation.
• api - runs created with POST /v1/rebalancing/runs.
• dashboard - runs created by a user in the Broker API dashboard.

Clients that parse initiated_from with a strict enum should accept dashboard before dashboard-created runs appear.

Affected Endpoints

• GET /v1/rebalancing/runs
• GET /v1/rebalancing/runs/{run_id}