fix(logging): give logs SSE stream a real cancel() hook and bump logBus listener cap

logs.ts used a TransformStream, which has no cancel() callback, so the
handler leaned only on write-error and abort-signal cleanup unlike the
requests/alerts SSE handlers. Switch to a ReadableStream with start/cancel
to match, and raise logBus's max listeners to 200 like requestEvents and
alertEvents to avoid MaxListenersExceededWarning under many concurrent
log-stream viewers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>