[mod] engines: migration of the individual cache solutions to EngineCache

The EngineCache class replaces all previously individual solutions for caches in
the context of the engines.

- demo_offline.py
- duckduckgo.py
- radio_browser.py
- soundcloud.py
- startpage.py
- wolframalpha_api.py
- wolframalpha_noapi.py

Search term to test most of the modified engines::

    !ddg !rb !sc !sp !wa test

    !ddg !rb !sc !sp !wa foo

For introspection of the DB, jump into developer environment and run command to
show cache state::

    $ ./manage pyenv.cmd bash --norc --noprofile
    (py3) python -m searx.enginelib cache state

    cache tables and key/values
    ===========================
    [demo_offline        ] 2025-04-22 11:32:50 count        --> (int) 4
    [startpage           ] 2025-04-22 12:32:30 SC_CODE      --> (str) fSOBnhEMlDfE20
    [duckduckgo          ] 2025-04-22 12:32:31 4dff493e.... --> (str) 4-128634958369380006627592672385352473325
    [duckduckgo          ] 2025-04-22 12:40:06 3e2583e2.... --> (str) 4-263126175288871260472289814259666848451
    [radio_browser       ] 2025-04-23 11:33:08 servers      --> (list) ['https://de2.api.radio-browser.info',  ...]
    [soundcloud          ] 2025-04-29 11:40:06 guest_client_id --> (str) EjkRJG0BLNEZquRiPZYdNtJdyGtTuHdp
    [wolframalpha        ] 2025-04-22 12:40:06 code         --> (str) 5aa79f86205ad26188e0e26e28fb7ae7
    number of tables: 6
    number of key/value pairs: 7

In the "cache tables and key/values" section, the table name (engine name) is at
first position on the second there is the calculated expire date and on the
third and fourth position the key/value is shown.

About duckduckgo: The *vqd coode* of ddg depends on the query term and therefore
the key is a hash value of the query term (to not to store the raw query term).

In the "properties of ENGINES_CACHE" section all properties of the SQLiteAppl /
ExpireCache and their last modification date are shown::

    properties of ENGINES_CACHE
    ===========================
    [last modified: 2025-04-22 11:32:27] DB_SCHEMA           : 1
    [last modified: 2025-04-22 11:32:27] LAST_MAINTENANCE    :
    [last modified: 2025-04-22 11:32:27] crypt_hash          : ca612e3566fdfd7cf7efe2b1c9349f461158d07cb78a3750e5c5be686aa8ebdc
    [last modified: 2025-04-22 11:32:30] CACHE-TABLE--demo_offline: demo_offline
    [last modified: 2025-04-22 11:32:30] CACHE-TABLE--startpage: startpage
    [last modified: 2025-04-22 11:32:31] CACHE-TABLE--duckduckgo: duckduckgo
    [last modified: 2025-04-22 11:33:08] CACHE-TABLE--radio_browser: radio_browser
    [last modified: 2025-04-22 11:40:06] CACHE-TABLE--soundcloud: soundcloud
    [last modified: 2025-04-22 11:40:06] CACHE-TABLE--wolframalpha: wolframalpha

These properties provide information about the state of the ExpireCache and
control the behavior.  For example, the maintenance intervals are controlled by
the last modification date of the LAST_MAINTENANCE property and the hash value
of the password can be used to detect whether the password has been changed (in
this case the DB entries can no longer be decrypted and the entire cache must be
discarded).

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

searx/engines/soundcloud.py

@@ -1,14 +1,23 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """SoundCloud is a German audio streaming service."""
+from __future__ import annotations
 
 import re
-from urllib.parse import quote_plus, urlencode
+import typing
 import datetime
 
+from urllib.parse import quote_plus, urlencode
+
 from dateutil import parser
 from lxml import html
 
 from searx.network import get as http_get
+from searx.enginelib import EngineCache
+
+if typing.TYPE_CHECKING:
+    import logging
+
+    logger: logging.Logger
 
 about = {
     "website": "https://soundcloud.com",
@@ -28,7 +37,6 @@ HTML frontend of the common WEB site.
 """
 
 cid_re = re.compile(r'client_id:"([^"]*)"', re.I | re.U)
-guest_client_id = ""
 results_per_page = 10
 
 soundcloud_facet = "model"
@@ -48,6 +56,10 @@ app_locale_map = {
     "sv": "sv",
 }
 
+CACHE: EngineCache
+"""Persistent (SQLite) key/value cache that deletes its values after ``expire``
+seconds."""
+
 
 def request(query, params):
 
@@ -55,6 +67,12 @@ def request(query, params):
     # - user_id=451561-497874-703312-310156
     # - app_version=1740727428
 
+    guest_client_id = CACHE.get("guest_client_id")
+    if guest_client_id is None:
+        guest_client_id = get_client_id()
+        if guest_client_id:
+            CACHE.set(key="guest_client_id", value=guest_client_id)
+
     args = {
         "q": query,
         "offset": (params['pageno'] - 1) * results_per_page,
@@ -104,12 +122,12 @@ def response(resp):
     return results
 
 
-def init(engine_settings=None):  # pylint: disable=unused-argument
-    global guest_client_id  # pylint: disable=global-statement
-    guest_client_id = get_client_id()
+def init(engine_settings):  # pylint: disable=unused-argument
+    global CACHE  # pylint: disable=global-statement
+    CACHE = EngineCache(engine_settings["name"])  # type:ignore
 
 
-def get_client_id() -> str:
+def get_client_id() -> str | None:
 
     client_id = ""
     url = "https://soundcloud.com"
@@ -143,4 +161,4 @@ def get_client_id() -> str:
         logger.info("using client_id '%s' for soundclud queries", client_id)
     else:
         logger.warning("missing valid client_id for soundclud queries")
-    return client_id
+    return client_id or None