tests: extract shared jellyfin test helpers and use real jellyfin in annotations test

tests/jellyfin-test-lib.nix

@@ -0,0 +1,20 @@
+{ pkgs, lib }:
+{
+  payloads = {
+    auth = pkgs.writeText "auth.json" (builtins.toJSON { Username = "jellyfin"; });
+    empty = pkgs.writeText "empty.json" (builtins.toJSON { });
+  };
+
+  helpers = ./jellyfin-test-lib.py;
+
+  jellyfinTestConfig =
+    { pkgs, ... }:
+    {
+      services.jellyfin.enable = true;
+      environment.systemPackages = with pkgs; [
+        curl
+        ffmpeg
+      ];
+      virtualisation.diskSize = lib.mkDefault (3 * 1024);
+    };
+}