Most Bambu Lab printers only allow one concurrent camera connection, but
GET /printers/{id}/camera/stream opened a fresh upstream per viewer.
Two browser tabs → second viewer fails or kicks the first off.
New MjpegBroadcaster (services/camera_fanout.py) owns one upstream per
printer and fans MJPEG chunks out to N subscribers. 5 s grace window
absorbs tab refreshes without reconnecting. Bounded subscriber queues
drop frames for slow viewers rather than blocking the broadcaster.
Audit-pass fixes:
- _stream_start_times set with setdefault() so stream_uptime reflects
the shared upstream's age, not the most-recent viewer's
- subscribe() retried once on RuntimeError to close a tiny grace race
- unsubscribe() returns post-removal count atomically so the detach log
no longer races with concurrent leavers
Permission gates unchanged; broadcaster has no FastAPI surface.
Tests: 13 broadcaster unit tests + 2 integration tests on /camera/stop.
External-camera path untouched.
