Add SD image pipeline, documentation overhaul, and fix module issues

- Add automatic SD image builds for Raspberry Pi via Forgejo Actions
- Enable binfmt emulation on cryodev-main for aarch64 cross-builds
- Add sd-image.nix module to cryodev-pi configuration
- Create comprehensive docs/ structure with installation guides
- Split installation docs into: first-install (server), reinstall, new-client (Pi)
- Add lib/utils.nix and apps/rebuild from synix
- Fix headplane module for new upstream API (tale/headplane)
- Fix various module issues (mailserver stateVersion, option conflicts)
- Add placeholder secrets.yaml files for both hosts
- Remove old INSTRUCTIONS.md (content moved to docs/)

hosts/cryodev-main/services/mailserver.nix

@@ -1,6 +1,4 @@
 {
-  config,
-  pkgs,
   outputs,
   constants,
   ...
@@ -21,7 +19,9 @@
         aliases = [ "postmaster" ];
       };
     };
-    certificateScheme = "acme-nginx";
-    sops = true;
+    x509.useACMEHost = constants.services.mail.fqdn;
   };
+
+  # ACME certificate for mail server
+  security.acme.certs.${constants.services.mail.fqdn} = { };
 }