The current frontend implementation closes the connection once `workflow_paused` SSE event is received and establish a new connection to subscribe new events. The implementation of `StreamsBroadcastChannel` sets initial `_last_id` to `0-0`, consumes streams from start and send `workflow_paused` event created before pauses to frontend, causing excessive connections being established.
This PR fixes the issue by setting initial id to `$`, which means only new messages are received by the subscription.
This PR introduces a `BroadcastChannel` abstraction with broadcasting and at-most once delivery semantics, serving as the communication component between celery worker and API server.
It also includes a reference implementation backed by Redis PubSub.
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
