[fix] make flask_babel.gettext() work in engine modules (L10n & threads)

incident:
  flask_babel.gettext() does not work in the engine modules.

cause:
  the request() and response() functions of the engine modules run in the
  processor, whose search() method runs in a thread and in the threads the
  context of the Flask app does not exist. The context of the Flask app is
  needed by the gettext() function for the L10n.

Solution:
  copy context of the Flask app into the threads. [1]

special case:
  We cannot equip the search() method of the processors with the decorator [1],
  because the decorator requires a context (Flask app) that does not yet exist
  at the time of the initialization of the processors (the initialization of the
  processors is part of the initialization of the Flask app).

[1] https://flask.palletsprojects.com/en/2.3.x/api/#flask.copy_current_request_context



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>