feat(devicebackup): UrBackup for the Windows laptops, Time Machine staging for the Macs

New devicebackup stack: UrBackup 2.5.x (pinned, watchtower off) in internet
mode on :55415 so laptops back up on the LAN and over the tailnet; web UI
only via Caddy + Authentik forward_auth (provider homelab-urbackup, pk 41).

MacBooks (tm-hub, tm-wif) use Time Machine on the host smbd, which already
owns :445. Shares are staged in /etc/samba/smb.conf.timemachine-pending and
not yet applied; README has the apply command and client setup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

stacks/devicebackup/stack.env

@@ -0,0 +1,11 @@
+# devicebackup stack — UrBackup server.
+TZ=America/Chicago
+
+# Run as ginnoir so /config/urbackup and /storage1/labdata/urbackup stay ginnoir-owned.
+PUID=1000
+PGID=1000
+
+# Samba password for the host-managed Time Machine shares (tm-hub, tm-wif).
+# Not read by the container; lives here so it is versioned with the stack (see README).
+TIMEMACHINE_SMB_USER=timemachine
+TIMEMACHINE_SMB_PASSWORD=Ey6gC0IL3rtm6nNL7oV3e08M