Currently, this allows the browser to subscribe to the response of the
asynchronous crawl after they add a new feed.
Eventually I will also use this in the main scheduled crawls. Right now,
it only upserts feed metadata based on the parsed feed.
Sets up a watch channel to send tracing lines from tracing-subscriber to
receivers in a axum handler which streams Server Sent Events to any
number of connected /log/stream clients.
