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/config/config.exs

@@ -0,0 +1,5 @@
+import Config
+
+config :logger, :console,
+  format: "$time $metadata[$level] $message\n",
+  metadata: [:request_id]

server/config/runtime.exs

@@ -0,0 +1,18 @@
+import Config
+
+# Get hostname for topic construction
+hostname = case :inet.gethostname() do
+  {:ok, hostname} -> List.to_string(hostname)
+  _ -> "unknown"
+end
+
+# Runtime configuration that can use environment variables
+config :systant, Systant.MqttClient,
+  host: System.get_env("SYSTANT_MQTT_HOST", "mqtt.home"),
+  port: String.to_integer(System.get_env("SYSTANT_MQTT_PORT", "1883")),
+  client_id: System.get_env("SYSTANT_CLIENT_ID", "systant"),
+  username: System.get_env("SYSTANT_MQTT_USERNAME"),
+  password: System.get_env("SYSTANT_MQTT_PASSWORD"),
+  stats_topic: System.get_env("SYSTANT_STATS_TOPIC", "systant/#{hostname}/stats"),
+  command_topic: System.get_env("SYSTANT_COMMAND_TOPIC", "systant/#{hostname}/commands"),
+  publish_interval: String.to_integer(System.get_env("SYSTANT_PUBLISH_INTERVAL", "30000"))