plex: restore Network=dankserver_appnet:alias=plex (line churn ate it)

The C5 probe failed for the wrong reason: plex.container lost its Network=
key in the same edit churn that ate tautulli's ContainerName. Plex ran on
the default podman network — published 32400 still worked (clients fine)
but tautulli could not resolve the plex alias, breaking the seeded
PMS_Web_Host=plex. Restore appnet membership; structure now parallel to
tautulli.container. Lesson recorded: after multi-hunk line edits on these
unit files, diff the whole file — key-order churn has eaten three keys
already.

dankserver/plex/quadlet/plex.container

@@ -6,6 +6,7 @@ Wants=network-online.target
 [Container]
 Image=lscr.io/linuxserver/plex:1.43.3.10896-cb3ebc72d-ls322
 ContainerName=plex
+Network=dankserver_appnet:alias=plex
 Environment=PUID=0
 Environment=PGID=0
 Environment=TZ=America/Los_Angeles