[mod] data: implement a simple currencies (SQL) database (#4836)

To reduce the memory footprint, this patch no longer loads the JSON data
completely into memory.  Instead, there is an SQL database based on
`ExpireCacheSQLite`.

The class CurrenciesDB is a simple DB application that encapsulates the
DB (queries and initialization) and provides convenient methods like
`name_to_iso4217` and `iso4217_to_name`.

Related:

- https://github.com/searxng/searxng/discussions/1892
- https://github.com/searxng/searxng/pull/3458#issuecomment-2900807671
- https://github.com/searxng/searxng/pull/4650

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

searx/data/core.py

@@ -0,0 +1,29 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# pylint: disable=missing-module-docstring
+from __future__ import annotations
+
+import pathlib
+
+from searx import logger
+from searx.cache import ExpireCacheCfg, ExpireCacheSQLite
+
+log = logger.getChild("data")
+
+data_dir = pathlib.Path(__file__).parent
+
+_DATA_CACHE: ExpireCacheSQLite = None  # type: ignore
+
+
+def get_cache():
+
+    global _DATA_CACHE  # pylint: disable=global-statement
+
+    if _DATA_CACHE is None:
+        _DATA_CACHE = ExpireCacheSQLite.build_cache(
+            ExpireCacheCfg(
+                name="DATA_CACHE",
+                # MAX_VALUE_LEN=1024 * 200,  # max. 200kB length for a *serialized* value.
+                # MAXHOLD_TIME=60 * 60 * 24 * 7 * 4,  # 4 weeks
+            )
+        )
+    return _DATA_CACHE