grafana: replace disk-usage-collector with prometheus-zfs-exporter

The custom disk-usage-collector shell script + minutely timer is replaced
by prometheus-zfs-exporter (pdf/zfs_exporter, packaged in nixpkgs as
services.prometheus.exporters.zfs). The exporter provides pool capacity
metrics (allocated/free/size) natively.

Partition metrics (/boot, /persistent, /nix) now use node_exporter's
built-in filesystem collector (node_filesystem_*_bytes) which already
runs and collects these metrics.

Also fixes a latent race condition in serviceMountWithZpool: the -mounts
service now orders after zfs-mount.service (which runs 'zfs mount -a'),
not just after pool import. Without this, the mount check could run
before datasets are actually mounted.

services/grafana/default.nix

@@ -5,7 +5,6 @@
     ./dashboard.nix
     ./exporters.nix
     ./jellyfin-annotations.nix
-    ./disk-usage-collector.nix
     ./llama-cpp-annotations.nix
     ./zfs-scrub-annotations.nix
   ];