[feat] engines: add duckduckgo web engine as alternative to html.duckduckgo.com

html.duckduckgo.com captchas all my IPs very fast. I figured out that using
duckduckgo.com works even if html.duckduckgo.com is captcha-ed, hence adding
support for duckduckgo.com's general web search here.

This implementation fetches the link to the first API page
(i.e. ``links.duckduckgo.com/d.js?...``) from duckduckgo.com and uses the ``n``
parameter of the API to fetch all subsequent pages.

This also means that it's not possible to immediately search for the third
page - the first and the second page would need to be loaded first.

The reason why we can't just normally use the `vqd` value is that the API URLs
require an additional parameter `dp` which seems generated at server-side, so we
can't build it ourselves and must scrape it from the HTML pages.

searx/engines/duckduckgo_web.py

@@ -0,0 +1,154 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""DuckDuckGo Web (general)
+
+This implementation fetches the link to the first API page
+(i.e. ``links.duckduckgo.com/d.js?...``) from duckduckgo.com and uses the ``n``
+parameter of the API to fetch all subsequent pages.
+
+This also means that it's not possible to immediately search for the third
+page - the first and the second page would need to be loaded first.
+
+The reason why we can't just normally use the `vqd` value is that the API URLs
+require an additional parameter `dp` which seems generated at server-side, so we
+can't build it ourselves and must scrape it from the HTML pages.
+"""
+
+import typing as t
+
+from urllib.parse import quote_plus
+from lxml import html
+
+from searx.utils import html_to_text, gen_useragent, extract_text, eval_xpath
+from searx.result_types import EngineResults
+from searx.enginelib import EngineCache
+from searx.network import get
+
+if t.TYPE_CHECKING:
+    from searx.extended_types import SXNG_Response
+    from searx.search.processors import OnlineParams
+
+about = {
+    "website": "https://duckduckgo.com/",
+    "wikidata_id": "Q12805",
+    "use_official_api": False,
+    "require_api_key": False,
+    "results": "JSON",
+}
+
+# engine dependent config
+categories = ["general"]
+paging = True
+_HTTP_User_Agent: str = gen_useragent()
+
+base_url = "https://duckduckgo.com"
+
+CACHE: EngineCache
+"""Cache to store the API URLs for combinations of (query, page)."""
+
+
+def setup(engine_settings: dict[str, str]):
+    global CACHE  # pylint:disable=global-statement
+    CACHE = EngineCache(engine_settings["name"])
+    return CACHE
+
+
+def _fetch_first_page_link(
+    query: str,
+    headers: dict[str, str],
+):
+    """Search for a::
+
+        <link id="deep_preload_link" rel="preload" as="script"
+              href="https://links.duckduckgo.com/d.js?q=rust&t=D&l=us-en&s=0&a=h_&ct=DE&vqd=VQD_VALUE&bing_market=en-US&p_ent=&ex=-1&dp=LONG_TOKEN
+        >
+
+    This points to the first page
+    """  # pylint:disable=line-too-long
+
+    cache_key = _cache_key(query, 1)
+    cached: str | None = CACHE.get(cache_key)
+    if cached:
+        return cached
+
+    resp = get(
+        url=f"{base_url}/?q={quote_plus(query)}&t=h_&ia=web",
+        headers=headers,
+        timeout=2,
+    )
+
+    if resp.status_code != 200:
+        logger.error("vqd: got HTTP %s from duckduckgo.com", resp.status_code)
+
+    dom = html.fromstring(resp.text)
+    first_page_link = extract_text(eval_xpath(dom, "//link[@id='deep_preload_link']/@href"))
+
+    if not first_page_link:
+        logger.error("vqd: failed to load first page JS url from ddg response (return empty string)")
+        return ""
+
+    logger.debug("got link to first page from duckduckgo.com request: '%s'", first_page_link)
+    CACHE.set(cache_key, first_page_link, expire=7200)
+
+    return first_page_link
+
+
+def _cache_key(query: str, pageno: int) -> str:
+    return f"nextpage_url|{query}|{pageno}"
+
+
+def request(query: str, params: "OnlineParams") -> None:
+
+    if len(query) >= 500:
+        # DDG does not accept queries with more than 499 chars
+        params["url"] = None
+        return
+
+    headers = params["headers"]
+
+    # The vqd value is generated from the query and the UA header.  To be able
+    # to reuse the vqd value, the UA header must be static.
+    headers["User-Agent"] = _HTTP_User_Agent
+    headers["Accept"] = "*/*"
+    headers["Referer"] = f"{base_url}/"
+    headers["Host"] = "duckduckgo.com"
+
+    # Sec-Fetch headers are required to not get blocked when sending a Firefox user agent
+    headers["Sec-Fetch-Dest"] = "script"
+    headers["Sec-Fetch-Mode"] = "no-cors"
+    headers["Sec-Fetch-Site"] = "same-site"
+
+    api_url = ""
+    if params["pageno"] > 1:
+        api_url = CACHE.get(_cache_key(query, params["pageno"]))
+    else:
+        api_url = _fetch_first_page_link(query, headers)
+
+    if not api_url:
+        params["url"] = None
+        return
+
+    params["url"] = api_url.replace("/d.js?", "/d.js?o=json&")
+
+    # TODO: support safesearch, timerange and engine traits  # pylint:disable=fixme
+
+
+def response(resp: "SXNG_Response"):
+    res = EngineResults()
+    res_json = resp.json()
+
+    for result in res_json["results"]:
+        if "u" not in result:
+            continue
+
+        res.add(res.types.MainResult(url=result["u"], title=result["t"], content=html_to_text(result["a"])))
+
+    # link to next page
+    next_page_path = res_json["results"][-1].get("n")
+    if next_page_path:
+        CACHE.set(
+            _cache_key(resp.search_params["query"], resp.search_params["pageno"] + 1),
+            base_url + next_page_path,
+            expire=60 * 60,
+        )
+
+    return res