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/api_task_controller.ex

@@ -5,6 +5,8 @@ defmodule PinchflatWeb.Api.V1.ApiTaskController do
 
   use PinchflatWeb, :controller
 
+  action_fallback PinchflatWeb.Api.V1.FallbackController
+
   alias Pinchflat.Repo
   alias Pinchflat.Tasks