quadlet: drop the userns identity model — rootless cannot map uid 997

Evidence from the failed first cutover: podman top shows container root
maps to dank (1000) under UserNS=host rootless (subuid range 100000:65536
cannot express 997), so PUID=997 fell back to root and PMS crash-looped on
write access to its 997:984-owned data root. Fleet model instead: PUID=0/
PGID=0 (container root = dank, same as jellyfin/copyparty data roots) and
deploy_plex chowns /var/lib/plexmediaserver to 1000:1000 inside the
first-cutover block. Zero bytes moved; rollback regains one chown -R
997:984 step, documented in Plex.md. Seam test pins PUID=0/PGID=0 and the
absence of any userns key.

dankserver/plex/quadlet/plex.container

@@ -1,14 +1,13 @@
 [Unit]
-Description=Plex Media Server (quadlet, userns=host)
+Description=Plex Media Server (quadlet, rootless fleet identity)
 After=network-online.target
 Wants=network-online.target
 
 [Container]
 Image=lscr.io/linuxserver/plex:1.43.3.10896-cb3ebc72d-ls322
 ContainerName=plex
-UserNS=host
-Environment=PUID=997
-Environment=PGID=984
+Environment=PUID=0
+Environment=PGID=0
 Environment=TZ=America/Los_Angeles
 Volume=/var/lib/plexmediaserver:/config
 Volume=/mnt/entropy:/mnt/entropy:ro