refactor(ssh): decentralize SSH configuration to per-host services

Restructures SSH trust relationships from global to host-specific configuration
for better locality of concern and principle of least privilege.

Changes:
- Collapse nixos/common/global/ssh/ back to ssh.nix (single-file module)
- Move internal host trust (fuchsia/viridian) to per-host services/ssh/
- Split BorgBase known hosts by repository (li9kg944 for fuchsia, r7ag7x1w for viridian)
- Add viridian SSH server config to accept backup connections from fuchsia
- Add fuchsia borgbackup passphrase for offsite backups
- Configure viridian to create /srv/borg-repo/fuchsia for remote backups

This enables the 3-2-1 backup strategy with fuchsia backing up to both viridian
(onsite) and BorgBase (offsite) with proper SSH authentication.

nixos/common/global/ssh.nix

@@ -0,0 +1,17 @@
+{...}: {
+  # Global SSH server configuration baseline
+  # Host-specific trust relationships are configured in each host's services/ssh/
+
+  services.openssh = {
+    enable = true;
+
+    settings = {
+      PermitRootLogin = "no";           # Disable root login for security
+      PasswordAuthentication = false;   # Require key-based authentication
+      LogLevel = "VERBOSE";             # Enhanced logging for security auditing
+    };
+
+    ports = [22];                       # Standard SSH port
+    openFirewall = true;                # Allow SSH through firewall
+  };
+}

nixos/common/global/ssh/default.nix

@@ -1,43 +0,0 @@
-{inputs, ...}: {
-  # SSH server configuration
-  services.openssh = {
-    enable = true;
-
-    settings = {
-      PermitRootLogin = "no";           # Disable root login for security
-      PasswordAuthentication = false;   # Require key-based authentication
-      LogLevel = "VERBOSE";             # Enhanced logging for security auditing
-    };
-
-    ports = [22];                       # Standard SSH port
-    openFirewall = true;                # Allow SSH through firewall
-  };
-
-  # Trusted host keys for internal infrastructure
-  programs.ssh.knownHosts = {
-    # Desktop workstation (fuchsia)
-    "fuchsia-ed25519" = {
-      hostNames = ["fuchsia"];
-      publicKeyFile = "${inputs.self}/nixos/fuchsia/ssh_host_ed25519_key.pub";
-    };
-    "fuchsia-rsa" = {
-      hostNames = ["fuchsia"];
-      publicKeyFile = "${inputs.self}/nixos/fuchsia/ssh_host_rsa_key.pub";
-    };
-
-    # Server (viridian)
-    "viridian-ed25519" = {
-      hostNames = ["viridian"];
-      publicKeyFile = "${inputs.self}/nixos/viridian/ssh_host_ed25519_key.pub";
-    };
-    "viridian-rsa" = {
-      hostNames = ["viridian"];
-      publicKeyFile = "${inputs.self}/nixos/viridian/ssh_host_rsa_key.pub";
-    };
-  };
-
-  # External backup provider (BorgBase)
-  programs.ssh.knownHostsFiles = [
-    ./borgbase_hosts
-  ];
-}

nixos/fuchsia/services/default.nix

@@ -7,6 +7,7 @@
     ./pipewire
     ./printing
     ./snapper
+    ./ssh
     ./udev
     ./xserver
   ];

nixos/fuchsia/services/ssh/borgbase_hosts

@@ -0,0 +1,3 @@
+li9kg944.repo.borgbase.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIMS3185JdDy7ffnr0nLWqVy8FaAQeVh1QYUSiNpW5ESq
+li9kg944.repo.borgbase.com ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCwHsO5g7kAEpqcK4bpHCUKYV1cKCUNwVEVsDQyfj7N8L92E21n+aEhIX2Nh/kFs1W9D/pgsWQBAbco9e/ORuagHrO8hUQtbda5Z31PAo4eipwP17VQr5rF3seaJJNFV72v89PGwMOWQwvoJte+yngC6PYGKJ+w63SRtflihAmf4xa5Tci/f6jbX6t32m2F3bnephVzQO6anGXvGPR8QYQXzSu/27+LaKnLd2Kugb1Ytbo0+6kioa60HWejIZ/mCrCHXYpi0jAllaYEuAsTqFWf/OFUHrKWwRAJD0TV43O1++vLlxY85oQxIgc4oUbm93dXmDBssrTnqqq2jqonteUr
+li9kg944.repo.borgbase.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBOstKfBbwVOYQh3J7X4nzd6/VYgLfaucP9z5n4cpSzcZAOKGh6jH8e1mhQ4YupthlsdPKyFFZ3pKo4mTaRRuiJo=

nixos/fuchsia/services/ssh/default.nix

@@ -0,0 +1,18 @@
+{inputs, ...}: {
+  # Trust viridian's host keys for SSH connections
+  programs.ssh.knownHosts = {
+    "viridian-ed25519" = {
+      hostNames = ["viridian"];
+      publicKeyFile = "${inputs.self}/nixos/viridian/ssh_host_ed25519_key.pub";
+    };
+    "viridian-rsa" = {
+      hostNames = ["viridian"];
+      publicKeyFile = "${inputs.self}/nixos/viridian/ssh_host_rsa_key.pub";
+    };
+  };
+
+  # Trust BorgBase repository (offsite backup target)
+  programs.ssh.knownHostsFiles = [
+    ./borgbase_hosts
+  ];
+}

nixos/viridian/services/default.nix

@@ -10,6 +10,7 @@
     ./murmur
     ./opengist
     ./snapper
+    ./ssh
     ./traefik
   ];
 }

nixos/viridian/services/ssh/default.nix

@@ -0,0 +1,29 @@
+{inputs, ...}: let
+  # Fuchsia's host key for backup authentication
+  fuchsiaHostKey = builtins.readFile (
+    "${inputs.self}/nixos/fuchsia/ssh_host_ed25519_key.pub"
+  );
+in {
+  # Trust fuchsia's host keys for SSH connections
+  programs.ssh.knownHosts = {
+    "fuchsia-ed25519" = {
+      hostNames = ["fuchsia"];
+      publicKeyFile = "${inputs.self}/nixos/fuchsia/ssh_host_ed25519_key.pub";
+    };
+    "fuchsia-rsa" = {
+      hostNames = ["fuchsia"];
+      publicKeyFile = "${inputs.self}/nixos/fuchsia/ssh_host_rsa_key.pub";
+    };
+  };
+
+  # Trust BorgBase repository (offsite backup target)
+  programs.ssh.knownHostsFiles = [
+    ./borgbase_hosts
+  ];
+
+  # Accept remote backups from fuchsia using host key authentication
+  users.users.root.openssh.authorizedKeys.keys = [
+    # Restrict fuchsia to only run borg serve in /srv/borg-repo
+    ''command="borg serve --restrict-to-path /srv/borg-repo",restrict ${fuchsiaHostKey}''
+  ];
+}