fix: mismatched time sources break birdseye idle heartbeat

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.

frigate/output/birdseye.py

@@ -854,7 +854,7 @@ class Birdseye:
                 coordinates = self.birdseye_manager.get_camera_coordinates()
                 self.requestor.send_data(UPDATE_BIRDSEYE_LAYOUT, coordinates)
         if self._idle_interval:
-            now = time.monotonic()
+            now = datetime.datetime.now().timestamp()
             is_idle = len(self.birdseye_manager.camera_layout) == 0
             if (
                 is_idle