[Proxmox LXC] Successful upgrade from Immich v2.7.5 to v3.0.1

Just sharing this in case it helps anyone else running Immich in a Proxmox LXC, rather than Docker.

I was running Immich v2.7.5 in a Proxmox LXC, installed via Community Scripts, with systemd services rather than Docker. I found the v3 upgrade guidance a bit confusing because a lot of the discussion seemed focused on Docker installs and Docker-specific changes before upgrading.

In my case, doing a regular update within the LXC console probably would have worked fine. But given the uncertainty, I took some precautionary steps first: I made a fresh Proxmox backup of the Immich LXC, made a separate PostgreSQL dump of the Immich database, checked that the dump was readable, backed up the Immich .env file, and checked that there was enough free space in the LXC.

After that, I ran the usual 'update' command inside the Immich LXC and the upgrade completed successfully.