Fix API controller tests: 404 handling, JSON encoding, media filtering

- Add FallbackController for proper 404 JSON responses on missing resources
- Add non-bang get_source/1 and get_media_item/1 for safe lookups
- Fix Task JSON encoder: remove Repo.preload call (not available in encoder scope),
  handle NotLoaded associations by returning nil
- Fix media item index: use list_media_items_for_source/1 instead of
  list_pending_media_items_for/1 which had overly restrictive filtering
- Add missing extras_directory to test config
- All 1007 tests pass

lib/pinchflat_web/controllers/api/v1/fallback_controller.ex

@@ -0,0 +1,18 @@
+defmodule PinchflatWeb.Api.V1.FallbackController do
+  @moduledoc """
+  Translates controller failures into JSON error responses.
+  """
+  use PinchflatWeb, :controller
+
+  def call(conn, {:error, :not_found}) do
+    conn
+    |> put_status(:not_found)
+    |> json(%{errors: %{detail: "Not found"}})
+  end
+
+  def call(conn, %Ecto.NoResultsError{} = _error) do
+    conn
+    |> put_status(:not_found)
+    |> json(%{errors: %{detail: "Not found"}})
+  end
+end