fix(webapp): use getRequestAbortSignal() for dashboard stream routes

Three dashboard-scoped stream routes were still passing request.signal
into realtimeStream.streamResponse. That signal is broken under
Remix+Express (see apps/webapp/CLAUDE.md, nodejs/node#55428 — the chain
is severed when Remix internally clones the Request), so when a user
closes their dashboard tab the signal never fires. The underlying
RedisRealtimeStreams.streamResponse loops while(!signal.aborted) over
XREAD BLOCK and only exits on its 15s inactivity timeout; the S2 path
keeps the upstream fetch open for up to its 60s wait window. In both
cases the per-request connection/buffer state sticks around longer
than it needs to, proportional to dashboard-session churn.

Thread getRequestAbortSignal() through:
  - resources/orgs/.../runs/$runParam/realtime/v1/streams/$runId/$streamId
  - resources/orgs/.../runs/$runParam/realtime/v1/streams/$runId/input/$streamId
  - resources/orgs/.../playground/realtime/v1/streams/$runId/$streamId
Each route picks up the Express res.on('close')-backed signal that
fires reliably when the downstream client disconnects.

Author: ericallam