Rebalancing run initiators documented in more detail

Broker API

The PortfolioRunInitiator schema description now spells out each run initiator:

• 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.

The accepted values are unchanged. Clients that validate this enum strictly should make sure they handle dashboard.

Affected Endpoints

• GET /v1/rebalancing/runs
• GET /v1/rebalancing/runs/{run_id}
• GET /v2/events/rebalancer