[feat] magnific: solve captcha to bypass botblocking

Magnific now employs a very simple to solve CAPTCHA before
all API requests (essentially summing up some numbers).

Python script for testing:
```python
import re
from lxml import html
import requests

headers = {
    'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:156.0) Gecko/20100101 Firefox/156.0',
    'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
    'Accept-Language': 'en-US,en;q=0.9',
    'Sec-GPC': '1',
    'Connection': 'keep-alive',
    'Upgrade-Insecure-Requests': '1',
    'Sec-Fetch-Dest': 'document',
    'Sec-Fetch-Mode': 'navigate',
    'Sec-Fetch-Site': 'same-origin',
    'Priority': 'u=0, i',
    'Pragma': 'no-cache',
    'Cache-Control': 'no-cache',
}

resp = requests.get(
    "https://www.magnific.com/api/regular/search?term=tree&filters%5Bai-generated%5D%5Bexcluded%5D=1&page=1&locale=en&filters%5Blicense%5D=free",
    headers=headers,
)
cookies = resp.cookies
print(resp.text)

_NUMBER_RE = re.compile("\d+")
_BM_VERIFY = re.compile(r"\"bm-verify\":\s*\"(.*?)\"")

doc = html.fromstring(resp.text)
script = doc.xpath("//script")[0].text
numbers = [int(m) for m in _NUMBER_RE.findall(script)]
bm_verify = _BM_VERIFY.search(resp.text)
if not bm_verify:
    exit(1)
bm_verify = bm_verify.group(1)
print("bm-verify: " + bm_verify)

solution = sum(numbers)
print(f"sum({numbers}) = {solution}")

resp = requests.post(
    "https://www.magnific.com/_sec/verify?provider=interstitial",
    json={"bm-verify": bm_verify, "pow": solution},
    headers=headers,
    cookies=cookies,
)

resp = requests.get(
    "https://www.magnific.com/api/regular/search?term=bird&filters%5Bai-generated%5D%5Bexcluded%5D=1&page=1&locale=en&filters%5Blicense%5D=free",
    headers=headers,
    cookies=resp.cookies,
)
print(resp.text)
```

searx/engines/magnific.py

@@ -4,11 +4,16 @@
 .. _Magnific: https://www.magnific.com
 """
 
+import re
 from urllib.parse import urlencode
 
 import typing as t
 
+from searx.enginelib import EngineCache
+from searx.exceptions import SearxEngineAPIException
+from searx.network import get, post
 from searx.result_types import EngineResults
+from searx.utils import eval_xpath_getindex, extract_text
 
 if t.TYPE_CHECKING:
     from searx.extended_types import SXNG_Response
@@ -26,7 +31,7 @@ about = {
 
 base_url = "https://www.magnific.com"
 
-categories = ["images"]
+categories = ["stock_images"]
 paging = True
 
 free_images_only = True
@@ -34,6 +39,47 @@ free_images_only = True
 Whether to only load images that may be used for free, without a Magnific account.
 """
 
+# regular expressions for solving the captcha
+_NUMBER_RE = re.compile(r"\d+")
+_BM_VERIFY_RE = re.compile(r"\"bm-verify\":\s*\"(.*?)\"")
+_CAPTCHA_COOKIE_NAME = "ak_bmsc"
+
+CACHE: EngineCache
+"""Cache for storing the cookie to bypass botblocking, obtained by solving a CAPTCHA."""
+
+
+def setup(engine_settings: dict[str, t.Any]):
+    global CACHE  # pylint: disable=global-statement
+    CACHE = EngineCache(engine_settings["name"])
+
+
+def _solve_captcha(original_resp: "SXNG_Response") -> "SXNG_Response":
+    """Solves the Magnific CAPTCHA by summing all numbers in the response.
+    E.g., the relevant part of the script that contains the numbers could look like
+    ``var i = 1789729970; var j = i + Number("1947" + "43160");``."""
+    doc = original_resp.html()
+    script = extract_text(eval_xpath_getindex(doc, "//script", 0)) or ""
+
+    bm_verify = _BM_VERIFY_RE.search(original_resp.text)
+    if not bm_verify:
+        raise SearxEngineAPIException("failed to extract bm verify token")
+    bm_verify = bm_verify.group(1)
+
+    # challenge is just adding a bunch of number together (some are strings, some are ints)
+    numbers = [int(m) for m in _NUMBER_RE.findall(script)]
+    solution = sum(numbers)
+
+    challenge_resp = post(
+        f"{base_url}/_sec/verify?provider=interstitial",
+        json={"bm-verify": bm_verify, "pow": solution},
+        cookies=original_resp.cookies,
+    )
+
+    resp = get(original_resp.url, headers=original_resp.search_params["headers"], cookies=challenge_resp.cookies)
+    # cookie is valid for 2 hours
+    CACHE.set(_CAPTCHA_COOKIE_NAME, challenge_resp.cookies[_CAPTCHA_COOKIE_NAME], expire=7200)
+    return resp
+
 
 def request(query: str, params: "OnlineParams") -> None:
     args = {"term": query, "filters[ai-generated][excluded]": 1, "page": params["pageno"], "locale": "en"}
@@ -41,10 +87,18 @@ def request(query: str, params: "OnlineParams") -> None:
         args["filters[license]"] = "free"
 
     params["headers"]["Referer"] = f"{base_url}/search"
+
+    # cookie that is obtained after solving the CAPTCHA
+    if cookie := CACHE.get(_CAPTCHA_COOKIE_NAME):
+        params["cookies"][_CAPTCHA_COOKIE_NAME] = cookie
+
     params["url"] = f"{base_url}/api/regular/search?{urlencode(args)}"
 
 
 def response(resp: "SXNG_Response"):
+    if resp.headers["content-type"] == "text/html":
+        resp = _solve_captcha(resp)
+
     res = EngineResults()
 
     result: dict[str, t.Any]  # TBH: dict[str, t.Any]