chore: add Mastodon maintenance page

Depends on pub-solar/pub.solar#15 being
merged and deployed.

hosts/flora-6/caddy.nix

@@ -4,7 +4,24 @@
   pkgs,
   self,
   ...
-}: {
+}: let
+  maintenanceMode = {
+    logFormat = lib.mkForce ''
+      output discard
+    '';
+    extraConfig = ''
+      root * /srv/www/pub.solar
+
+      error * "Scheduled Maintenance" 503
+
+      handle_errors {
+        root * /srv/www/pub.solar
+        rewrite * /maintenance/index.html
+        file_server
+      }
+    '';
+  };
+in {
   systemd.tmpfiles.rules = [
     "d '/data/srv/www/os/download/' 0750 hakkonaut hakkonaut - -"
   ];
@@ -79,6 +96,9 @@
           redir https://pub.solar{uri}
         '';
       };
+
+      "mastodon.pub.solar" = maintenanceMode;
+
       "auth.pub.solar" = {
         logFormat = lib.mkForce ''
           output discard