[fix] issues when launching a local development server

A local development server can be launched by one of these command lines::

    $ flask --app searx.webapp run
    $ python -m searx.webapp

The different ways of starting the server should lead to the same result, which
is generally the case.  However, if the modules are reloaded after code
changes (reload option), it must be avoided that the application is initialized
twice at startup.  We have already discussed this in 2022 [1][2].

Further information on this topic can be found in [3][4][5].

To test a bash in the ./local environment was started and the follwing commands
had been executed::

    $ ./manage pyenv.cmd bash --norc --noprofile
    (py3) SEARXNG_DEBUG=1 flask --app searx.webapp run --reload
    (py3) SEARXNG_DEBUG=1 python -m searx.webapp

Since the generic parts of the docs also initialize the app to generate doc from
it, the build of the docs was also tested::

    $ make docs.clean docs.live

[1] https://github.com/searxng/searxng/pull/1656#issuecomment-1214198941
[2] https://github.com/searxng/searxng/pull/1616#issuecomment-1206137468
[3] https://flask.palletsprojects.com/en/stable/api/#flask.Flask.run
[4] https://github.com/pallets/flask/issues/5307#issuecomment-1774646119
[5] https://stackoverflow.com/a/25504196

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

searx/search/checker/background.py

@@ -10,7 +10,7 @@ from typing import Any, Dict, List, Literal, Optional, Tuple, TypedDict, Union
 
 import redis.exceptions
 
-from searx import logger, settings, searx_debug
+from searx import logger, settings, sxng_debug
 from searx.redisdb import client as get_redis_client
 from searx.exceptions import SearxSettingsException
 from searx.search.processors import PROCESSORS
@@ -139,7 +139,7 @@ def initialize():
         signal.signal(signal.SIGUSR1, _signal_handler)
 
     # special case when debug is activate
-    if searx_debug and settings['checker']['off_when_debug']:
+    if sxng_debug and settings['checker']['off_when_debug']:
         logger.info('debug mode: checker is disabled')
         return