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/rel/env.sh.eex

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Configure environment for release
+export MIX_ENV=prod
+export RELEASE_DISTRIBUTION=none
+export RELEASE_NODE=nonode@nohost