[fix] sqlitedb: create DB Schema (DDL) during app initialization (hardening) (#6187)

The initialization of the DB schema ("base schema") has so far been done on
demand, which causes race conditions with competing threads and processes.

The DDL statements for creating the "base schema" are now executed as part of
the initialization of the app.

Further improvements were made to harden the database applications:

- Wikidata & Radio-Browser engine perform their initialization only once (so far
  the initialization was carried out in each thread/process).

- If multiple processes try to set DB's WAL mode when opening the DB at the same
  time, this usually leads to another race condition, which is now also caught.

Related:

- https://github.com/searxng/searxng/issues/6181#issuecomment-4586705

Closes: #6181

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

searx/search/processors/abstract.py

@@ -3,6 +3,7 @@
 
 import typing as t
 
+import os
 import logging
 import threading
 from abc import abstractmethod, ABC
@@ -154,7 +155,9 @@ class EngineProcessor(ABC):
         try:
             init_ok = self.engine.init(eng_setting)
         except Exception:  # pylint: disable=broad-except
-            logger.exception("Init method of engine %s failed due to an exception.", self.engine.name)
+            logger.exception(
+                f"(PID {os.getpid()}) Init method of engine %s failed due to an exception.", self.engine.name
+            )
             init_ok = False
         # In older engines, None is returned from the init method, which is
         # equivalent to indicating that the initialization was successful.