[Enhancement] Custom media lifecycle scripts (#219)

* Namespaced notification modules under lifecycle

* Added a JSON encoder for all the main model types

* Added startup task to create user script file

* Hook up user script event to media download

* Hooked up media deletion user script

* Added jq to docker deps

* Updated README

lib/pinchflat/lifecycle/user_scripts/command_runner.ex

@@ -0,0 +1,76 @@
+defmodule Pinchflat.Lifecycle.UserScripts.CommandRunner do
+  @moduledoc """
+  Runs custom user commands commands using the `System.cmd/3` function
+  """
+
+  require Logger
+
+  alias Pinchflat.Utils.CliUtils
+  alias Pinchflat.Utils.FilesystemUtils
+  alias Pinchflat.Lifecycle.UserScripts.UserScriptCommandRunner
+
+  @behaviour UserScriptCommandRunner
+
+  @event_types [
+    :media_downloaded,
+    :media_deleted
+  ]
+
+  @doc """
+  Runs the user script command for the given event type. Passes the event
+  and the encoded data to the user script command.
+
+  This function will succeed in almost all cases, even if the user script command
+  failed - this is because I don't want bad scripts to stop the whole process.
+  If something fails, it'll be logged.
+
+  The only things that can cause a true failure are passing in an invalid event
+  type or if the passed data cannot be encoded into JSON - both indicative of
+  failures in the development process.
+
+  Returns :ok
+  """
+  @impl UserScriptCommandRunner
+  def run(event_type, encodable_data) when event_type in @event_types do
+    case backend_executable() do
+      {:ok, :no_executable} ->
+        :ok
+
+      {:ok, executable_path} ->
+        {:ok, encoded_data} = Phoenix.json_library().encode(encodable_data)
+
+        {output, exit_code} =
+          CliUtils.wrap_cmd(
+            executable_path,
+            [to_string(event_type), encoded_data],
+            [],
+            logging_arg_override: "[suppressed]"
+          )
+
+        handle_output(output, exit_code)
+    end
+  end
+
+  def run(event_type, _encodable_data) do
+    raise ArgumentError, "Invalid event type: #{inspect(event_type)}"
+  end
+
+  defp handle_output(output, exit_code) do
+    Logger.debug("Custom lifecycle script exit code: #{exit_code} with output: #{output}")
+
+    :ok
+  end
+
+  defp backend_executable do
+    base_dir = Application.get_env(:pinchflat, :extras_directory)
+    filepath = Path.join([base_dir, "user-scripts", "lifecycle"])
+
+    if FilesystemUtils.exists_and_nonempty?(filepath) do
+      {:ok, filepath}
+    else
+      Logger.warning("User scripts lifecyle file either not present or is empty. Skipping.")
+
+      {:ok, :no_executable}
+    end
+  end
+end