feat(ipa): add reusable declarative FreeIPA client module

Adds modules/ipa/client.nix — a parameterized module that joins a NixOS host
to the sweet.home FreeIPA domain without ipa-client-install. It configures
security.ipa (SSSD, Kerberos, PAM, NSSwitch) and places a pre-provisioned host
keytab via sops-nix binary secret so enrollment is fully reproducible from the
flake.

- variables.nix: adds ipaServer (FQDN of the FreeIPA KDC; security.ipa.server
  requires a hostname, not an IP, for Kerberos/TLS)
- certs/ipa-ca.crt: placeholder for the IPA CA public certificate (operator
  replaces with: curl http://<ipa-server>/ipa/config/ca.crt)
- secrets/nix-cache.keytab: placeholder binary sops file (operator replaces
  with the encrypted keytab after ipa host-add + ipa-getkeytab)
- .sops.yaml: adds creation rule for secrets/nix-cache.keytab (same recipients
  as secrets/nix-cache.yaml)
- hosts/nix-cache/host.nix: imports the IPA client module; adds
  networking.domain so the host's FQDN resolves correctly

Module header documents the three operator steps needed per host before deploy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/ipa/client.nix

@@ -0,0 +1,73 @@
+# Fully declarative FreeIPA domain membership.
+#
+# Configures security.ipa (SSSD, Kerberos, PAM, NSSwitch) and places a
+# pre-provisioned host keytab via sops-nix so no imperative ipa-client-install
+# step is needed after deployment.
+#
+# Usage (in a host.nix imports list):
+#   (import ../../modules/ipa/client.nix {
+#     keytabSopsFile = ../../secrets/nix-cache.keytab;
+#     caCertFile     = ../../certs/ipa-ca.crt;
+#   })
+#
+# One-time operator setup per host (do this BEFORE deploying):
+#
+#   1. Fetch the IPA CA certificate (public — safe to commit):
+#        curl -o certs/ipa-ca.crt http://<ipa-server>/ipa/config/ca.crt
+#      Replace the placeholder at certs/ipa-ca.crt and commit it.
+#
+#   2. On the FreeIPA server, add the host and generate a keytab:
+#        ipa host-add <fqdn> --ip-address=<ip>
+#        ipa-getkeytab -s <ipa-server> -p host/<fqdn> -k /tmp/<host>.keytab
+#
+#   3. sops-encrypt the keytab as a binary secret from your admin machine:
+#        sops -e --input-type binary /tmp/<host>.keytab \
+#          > secrets/<host>.keytab
+#      Add secrets/<host>.keytab to .sops.yaml with the host's age key as a
+#      recipient (see the nix-cache.keytab entry for the pattern), then run:
+#        scripts/secrets/sync-host-keys.sh <target>   # if not done yet
+#        sops updatekeys secrets/<host>.keytab
+#      Commit the encrypted file.
+#
+#   4. nixos-rebuild switch (or create-proxmox-resource.sh) — no further
+#      manual enrollment steps required.
+#
+# vars dependencies: homeDomain, ipaServer
+
+{ keytabSopsFile, caCertFile }:
+{ config, lib, pkgs, vars, ... }:
+
+let
+  realm = lib.strings.toUpper vars.homeDomain;
+  fqdn = "${config.networking.hostName}.${vars.homeDomain}";
+  # "sweet.home" -> "dc=sweet,dc=home"
+  basedn = lib.strings.concatMapStringsSep "," (c: "dc=${c}") (lib.strings.splitString "." vars.homeDomain);
+  # security.ipa.certificate expects a derivation (package), not a raw path.
+  caCertPkg = pkgs.writeText "ipa-ca.crt" (builtins.readFile caCertFile);
+in
+{
+  security.ipa = {
+    enable = true;
+    domain = vars.homeDomain;
+    realm = realm;
+    server = vars.ipaServer;
+    certificate = caCertPkg;
+    basedn = basedn;
+    ipaHostname = fqdn;
+    offlinePasswords = true;
+    cacheCredentials = true;
+  };
+
+  # Host keytab: pre-provisioned on the IPA server, sops-encrypted binary.
+  # Placed at /etc/krb5.keytab before SSSD starts so the host authenticates
+  # to IPA without running ipa-client-install.
+  sops.secrets."ipa-host-keytab" = {
+    sopsFile = keytabSopsFile;
+    format = "binary";
+    path = "/etc/krb5.keytab";
+    owner = "root";
+    group = "root";
+    mode = "0600";
+    restartUnits = [ "sssd.service" ];
+  };
+}