Restructure as monorepo and add flake packages/apps

- Move Elixir code to server/ subdirectory for monorepo structure
- Update flake.nix to provide packages and apps outputs for nix run support
- Update nix/package.nix to accept src parameter instead of fetchgit
- Add NixOS module export for easy consumption

Now supports: nix run, nix build, and nix develop from git repo

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/test/systant_test.exs

@@ -0,0 +1,8 @@
+defmodule SystantTest do
+  use ExUnit.Case
+  doctest Systant
+
+  test "greets the world" do
+    assert Systant.hello() == :world
+  end
+end

server/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()