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>

nix/package.nix

@@ -1,18 +1,14 @@
 {
   lib,
   beamPackages,
-  fetchgit,
+  src,
 }:
 
 beamPackages.mixRelease rec {
   pname = "systant";
   version = "0.1.0";
 
-  src = fetchgit {
-    url = "https://git.ryanpandya.com/ryan/systant.git";
-    rev = "54f8d23945bcea17127631a81f8ac318bf7047a6";
-    sha256 = "sha256-1cRfSoH+JdO4a7q4hRZSkoDMk2wMCYRIyCIN56FSUgg=";
-  };
+  inherit src;
 
   # Mix dependencies will be automatically fetched and cached by Nix
   mixFodDeps = beamPackages.fetchMixDeps {