refactor(viridian): migrate service data to dedicated BTRFS subvolumes

Migrate from path-based persistence (/persist/var/lib/*) to dedicated
BTRFS subvolumes for better data isolation and snapshot capabilities.

- Move valuable user-facing services to /srv/* with srv-* subvolumes:
  - forgejo: git repositories and database
  - opengist: paste data
  - minecraft: game world data
  - lighttpd: static web content
  - containers: OCI container volumes

- Update home directory to use hm-sajenim subvolume on viridian disk
- Remove jupyterhub service (no longer in use)
- Update borgbackup paths to match new service locations
- Follow upstream service defaults where possible for maintainability

Services kept on /persist (disposable state):
- traefik, crowdsec, murmur

nixos/viridian/hardware-configuration.nix

@@ -52,17 +52,6 @@
     fsType = "vfat";
   };
 
-  fileSystems."/srv/multimedia" = {
-    device = "/dev/disk/by-label/multimedia";
-    fsType = "ext4";
-  };
-
-  fileSystems."/srv/multimedia/containers" = {
-    device = "/dev/disk/by-label/data";
-    fsType = "btrfs";
-    options = ["subvol=containers" "compress=zstd"];
-  };
-
   swapDevices = [
     {
       device = "/swap/swapfile";