Add Attic binary cache and containerize gitea runner

Replace nix-serve-only setup with Attic for managed binary caching with
upstream filtering and GC. Move gitea actions runner from host into an
isolated NixOS container with private networking. nix-serve kept alongside
Attic during migration.

common/server/atticd.nix

@@ -0,0 +1,31 @@
+{ config, lib, ... }:
+
+{
+  config = lib.mkIf (config.thisMachine.hasRole."binary-cache") {
+    services.atticd = {
+      enable = true;
+      environmentFile = config.age.secrets.atticd-credentials.path;
+      settings = {
+        listen = "[::]:28338";
+
+        chunking = {
+          nar-size-threshold = 64 * 1024; # 64 KiB
+
+          # The preferred minimum size of a chunk, in bytes
+          min-size = 16 * 1024; # 16 KiB
+
+          # The preferred average size of a chunk, in bytes
+          avg-size = 64 * 1024; # 64 KiB
+
+          # The preferred maximum size of a chunk, in bytes
+          max-size = 256 * 1024; # 256 KiB
+        };
+
+        compression.type = "zstd";
+        garbage-collection.default-retention-period = "6 months";
+      };
+    };
+
+    age.secrets.atticd-credentials.file = ../../secrets/atticd-credentials.age;
+  };
+}