frigate: Enable RTMP on Back Yard camera

I couldn't get RTMP to work on the Back Yard camera because the `ffmpeg`
process kept crashing:

```
ffmpeg.back_yard.clips_rtmp    ERROR   : av_interleaved_write_frame(): Connection reset by peer
ffmpeg.back_yard.clips_rtmp    ERROR   : [flv @ 0x5562090c8ec0] Failed to update header with correct duration.
ffmpeg.back_yard.clips_rtmp    ERROR   : [flv @ 0x5562090c8ec0] Failed to update header with correct filesize.
ffmpeg.back_yard.clips_rtmp    ERROR   : Error writing trailer of rtmp://127.0.0.1/live/back_yard: Connection reset by peer
watchdog.back_yard             INFO    : Terminating the existing ffmpeg process...
watchdog.back_yard             INFO    : Waiting for ffmpeg to exit gracefully...
```

I thought increasing the value of `--shm-size` argument for `podman`
would help, but even going as high as 1024 mebibytes did not resolve the
problem.

Ultimately, I decided that it is not really necessary to view the full
4k stream in real time.  The back yard camera supports three streams, so
I set them all up for different roles.  I briefly considered using a
single 1080p stream for both object detection and RTMP streaming, but
this consumed considerable CPU time, so I decided against it for now.  I
may re-evaluate that option if I decide to purchase a TPU.

group_vars/frigate/main.yml

@@ -46,13 +46,18 @@ frigate_cameras:
   back_yard:
     ffmpeg:
       inputs:
+      # 640x480 for object detection
       - path: rtsp://frigate:{{ vault_frigate_password_back_yard }}@172.30.0.212/cam/realmonitor?channel=1&subtype=1
         roles:
         - detect
+      # 4k for recording
       - path: rtsp://frigate:{{ vault_frigate_password_back_yard }}@172.30.0.212/cam/realmonitor?channel=1&subtype=0
         roles:
         - clips
-        #- rtmp
+      # 1080p for streaming viaHome Assistant
+      - path: rtsp://frigate:{{ vault_frigate_password_back_yard }}@172.30.0.212/cam/realmonitor?channel=1&subtype=2
+        roles:
+        - rtmp
     height: 480
     width: 640
     objects:
@@ -64,9 +69,9 @@ frigate_cameras:
     clips:
       enabled: true
     rtmp:
-      enabled: false
+      enabled: true
     snapshots:
       enabled: true
       retain:
          default: 365
-frigate_shm_size: 100
+frigate_shm_size: 256