nixos: describe asxpio-services as a NixOS system

asxpio-services is to move from Debian with Docker to NixOS with
Podman. This is the target system; nothing is switched over yet.

One flake holds what was spread over cloud-init, Ansible roles and
hand-made units: network and WireGuard, users and SSH restrictions,
firewall, the weekly image prune, and the four container stacks
(Paper and Fossils Minecraft, SillyTavern, the Forgejo runner), each
as a systemd unit under Podman. The runner keeps running as a
container and starts jobs through Podman's Docker-compatible socket.

Secrets (WireGuard keys, RCON and CurseForge keys) are encrypted with
sops for the admin's age key and the server's SSH host key, so the
repository holds no plaintext secret and CI needs none of them.

Containers that own data wait for /opt/.restored. On a freshly
installed disk a game server would otherwise generate a new world
exactly where the restore is about to land.

Tried on a throwaway server (.#rehearsal): install with nixos-anywhere
keeping a given host key, secret decryption, all stacks, a real Paper
server stopping with a full save, what CI jobs need from the engine
(short image names, the nix-store volume, service containers by name,
a kaniko build, the socket inside a job), an update over SSH and a
reboot.

migration/ has the scripts that move the data through object storage,
and a workflow builds the system on every push and switches the host
on main.

Assisted-by: Claude:fable-5-2

nixos/.sops.yaml

@@ -0,0 +1,13 @@
+# Who can decrypt what. Hosts decrypt with their SSH host key (converted
+# to age with ssh-to-age); the admin key edits everything.
+keys:
+  - &admin age19xvmm6j3h2u6mc3w6k65ufry4p5aa5lwu96p55h3e42p9wfzwc0qp2w7pk
+  - &asxpio_services age15ws8alg3ftsgmzvqxft09556g87s4n8c8v39nmvhaz6k50gp4e7se8acmg
+  - &rehearsal age1wdgvn9yjjjgqq0h7pysy9e0e74qag4v6t82qyspk2685c550vq3spg0yjj
+creation_rules:
+  - path_regex: secrets/asxpio-services\.yaml$
+    key_groups:
+      - age: [*admin, *asxpio_services]
+  - path_regex: secrets/rehearsal\.yaml$
+    key_groups:
+      - age: [*admin, *rehearsal]