feat(youko): serve restic backups over an append-only REST server

Run restic-rest-server behind caddy (append-only, private repos, htpasswd
from agenix) and register youko's host key centrally.

hosts/12kingdoms-youko/default.nix

@@ -36,10 +36,41 @@ in
     repository = "/data/backups/restic/youko";
     snapshotSource = "/btr_pool/@persistent";
     requiresMountsFor = "/data/backups";
+    exclude = [
+      # regenerable, huge, or unsuitable for file-level backup
+      #
+      # the whole podman storage tree: the image layers are re-pullable, and its
+      # storage DB is not consistent when copied from a running podman.
+      # (uptime-kuma uses a named volume under here; its data is not wanted.)
+      "var/lib/containers"
+      "var/lib/microvms"
+      "var/lib/libvirt"
+      "nfs"
+      "var/cache"
+      "var/tmp"
+      "var/log"
+      "*.qcow2"
+    ];
   };
 
   modules.btrbk.enable = true;
 
+  # The restic REST server the desktops push their backups to. Append-only, so
+  # a compromised client still cannot delete history; private repos, so each
+  # client only reaches the repository named after its user. Credentials come
+  # from agenix; caddy terminates TLS in front of it.
+  services.restic.server = {
+    enable = true;
+    listenAddress = "127.0.0.1:8000";
+    dataDir = "/data/backups/rest-server";
+    appendOnly = true;
+    privateRepos = true;
+    "htpasswd-file" = config.age.secrets."restic-rest-htpasswd".path;
+  };
+
+  # repositories live on the HDD, which has to be mounted first
+  systemd.services.restic-rest-server.unitConfig.RequiresMountsFor = "/data/backups";
+
   boot.kernelParams = [
     # Use transparent huge pages on demand (madvise) instead of a fixed 1G hugetlb
     # pool (cannot be overcommitted/shared; it stranded memory and blocked

hosts/12kingdoms-youko/homelab-services/caddy.nix

@@ -63,6 +63,11 @@ in
       encode zstd gzip
       reverse_proxy http://localhost:3303
     '';
+    # the restic REST server the backup clients push to
+    virtualHosts."backup.writefor.fun".extraConfig = ''
+      ${hostCommonConfig}
+      reverse_proxy http://localhost:8000
+    '';
     virtualHosts."transmission.writefor.fun".extraConfig = ''
       ${hostCommonConfig}
       encode zstd gzip

vars/networking.nix

@@ -187,6 +187,8 @@ rec {
           # Define the root user's host key for remote builders, so that nix can verify all the remote builders
 
           aquamarine.publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEOXFhFu9Duzp6ZBE288gDZ6VLrNaeWL4kDrFUh9Neic root@aquamarine";
+          # the restic backup server
+          youko.publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEs8b3CmxM+utXZVKvJBssXBupBXvvzjHP40HcY7PSyZ root@youko";
           # ruby.publicKey = "";
           # kana.publicKey = "";