Slice 1 of the #392 hybrid live-refresh work. Ships the in-process pub/sub
bus and the SSE subscriber endpoint; no producers wired yet, so the stream
emits only heartbeats today. Verifiable in isolation by curl-ing the
endpoint with a valid Bearer token — the connection opens, ": heartbeat"
lines arrive every 15s, the connection closes cleanly on client disconnect.
eventbus.Bus is a small fan-out broadcaster: subscribers register through
Subscribe (returns a receive channel + an unsubscribe closure), writers
call Publish, and the bus drops events for any subscriber whose buffer is
full rather than blocking the writer. No persistence — clients are
expected to resync via normal /api/* fetches on (re)connect.
The SSE handler emits an initial ": connected" comment so the client sees
the connection open immediately, then forwards events whose UserID matches
the authenticated user (or is empty for broadcast). Heartbeat comments
keep proxy connections alive. Context cancellation cleanly tears down the
subscription on client disconnect.
Producers (likes, request status, quarantine, scanner, playlist mutations)
land in subsequent slices.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
