fix: mismatched time sources break birdseye idle heartbeat (#22466)

The idle heartbeat check in BirdsEyeOutputProcess.update() compares
time.monotonic() (seconds since an arbitrary point, typically boot)
against last_output_time which is set from datetime.datetime.now().timestamp()
(Unix epoch seconds).

These are completely different time bases. The subtraction produces a
large negative number, so the idle heartbeat condition can never be
satisfied. This means birdseye stops sending frames when all cameras
go idle, instead of continuing at the configured idle_heartbeat_fps.

Use datetime.datetime.now().timestamp() consistently for both the
heartbeat check and the output time tracking.