Fix lxc-* hosts having no host-key pre-seeding mechanism at all

The real root cause behind the original nix-cache 502, traced all the way
through: modules/installer/host-keys.nix (which NIXOS_HOST_KEYS_DIR=...
--impure actually wires up) is only ever imported by the installer's own
modules/installer/common.nix -- modules/platforms/lxc.nix, which every
real lxc-* host build actually uses, never imported anything like it.
docs/auto-installer.md previously claimed NIXOS_HOST_KEYS_DIR bakes a key
into lxc-* tarballs "the same way it does for the ISO/PXE installer
images" -- that was never actually true; I wrote it without verifying the
mechanism existed for lxc.nix specifically.

In practice this meant every lxc-* container booted with a freshly
self-generated SSH host key that could never match whatever .sops.yaml
actually trusts for that target, so *every* secret -- not just
cache-priv-key -- silently failed to decrypt. No error surfaces in the
boot log for this: the activation step that installs secrets only runs
on a genuinely fresh first activation and silently no-ops once
/run/current-system already exists, so by the time anyone looks the
window has closed. Found by manually invoking sops-install-secrets
directly: "Error getting data key: 0 successful groups required, got 0".

Fixed by giving modules/platforms/lxc.nix the same key-baking mechanism
the installer has, but keyed to its own exact flake target and placing
the key directly at /etc/ssh/ssh_host_ed25519_key (no copy step to stage
for, unlike the installer's /etc/host-keys/ staging area -- an lxc-*
tarball has no install step). The target name comes in via
specialArgs.flakeTarget (new, set by flake.nix's mkTarget) rather than
being read back from config.environment.etc."flake-target" -- reading
that back from within a module that also contributes to
environment.etc is circular (confirmed: "infinite recursion
encountered").

Verified live end-to-end against the real test container (lxc-nix-cache,
VMID 100 on pve.sweet.home): destroyed it, rebuilt the tarball fresh with
the fix, recreated it, and confirmed /run/secrets/ now has all three
secrets this host needs (beszel-token, cache-priv-key, nix-github-token),
nix-serve is active (running), and curl http://localhost/nix-cache-info
succeeds both directly and through nginx.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

modules/platforms/lxc.nix

@@ -1,5 +1,44 @@
-{ lib, modulesPath, ... }:
+{ lib, modulesPath, flakeTarget, ... }:
 
+let
+  # Bakes this exact flake target's pre-generated SSH host key straight
+  # into /etc/ssh/ -- mirrors modules/installer/host-keys.nix's
+  # builtins.getEnv pattern (impure and empty under normal `nix
+  # build`/`nix eval`, so this is a no-op unless explicitly opted into
+  # with NIXOS_HOST_KEYS_DIR=... --impure), but places the key directly
+  # rather than staging it under /etc/host-keys/ for a later manual copy
+  # -- this is the whole system for a `lxc-*` host, built straight to a
+  # pct-restorable tarball with no install step, so there's no later copy
+  # step to stage for.
+  #
+  # Without this, config.system.build.tarball's built-in system just
+  # generates a fresh host key at first boot like any other host would --
+  # but sops-nix derives its decryption key from *this* file, and
+  # .sops.yaml only trusts whatever key scripts/sync-host-keys.sh already
+  # registered for this exact target name. A freshly-generated key can
+  # never match that, so every secret (including this host's own login)
+  # permanently fails to decrypt. Confirmed live: sops-install-secrets
+  # errored with "Error getting data key: 0 successful groups required,
+  # got 0" -- the container's actual host key's age fingerprint didn't
+  # match the one registered in .sops.yaml at all.
+  hostKeysDirStr = builtins.getEnv "NIXOS_HOST_KEYS_DIR";
+  hasHostKeysDir = hostKeysDirStr != "" && builtins.pathExists hostKeysDirStr;
+  hostKeysDir = /. + hostKeysDirStr;
+
+  # flakeTarget ("${platform}-${buildType}") comes in via specialArgs from
+  # flake.nix's mkTarget -- exactly the name scripts/sync-host-keys.sh
+  # registers keys under. Deliberately not read back from
+  # config.environment.etc."flake-target" (which is set to the same value)
+  # -- this module also *contributes* to environment.etc below, and a
+  # module reading the merged value of an option it's still defining is a
+  # circular dependency (confirmed: "infinite recursion encountered").
+  privKeyFile = hostKeysDir + "/${flakeTarget}_ssh_host_ed25519_key";
+  pubKeyFile = hostKeysDir + "/${flakeTarget}_ssh_host_ed25519_key.pub";
+  hasKeyForThisTarget =
+    hasHostKeysDir
+    && builtins.pathExists privKeyFile
+    && builtins.pathExists pubKeyFile;
+in
 {
   # LXC containers share the host kernel — Proxmox starts them by exec'ing
   # /sbin/init directly, no bootloader/initrd involved — and Proxmox has its
@@ -35,4 +74,15 @@
   # for the same reason; it just doesn't disable NetworkManager itself,
   # which modules/common/configuration.nix enables for every host.
   networking.networkmanager.enable = lib.mkForce false;
+
+  environment.etc = lib.mkIf hasKeyForThisTarget {
+    "ssh/ssh_host_ed25519_key" = {
+      source = privKeyFile;
+      mode = "0600";
+    };
+    "ssh/ssh_host_ed25519_key.pub" = {
+      source = pubKeyFile;
+      mode = "0644";
+    };
+  };
 }