refactor(search): import REQUEST_TIMEOUT from constants in providers.py (#4331)

providers.py redefined REQUEST_TIMEOUT = 20 locally, shadowing the same
value in src/constants.py and risking drift if the constant is bumped.
Import it from src.constants and drop the local copy; same value, one
source of truth.

Closes #4329

services/search/providers.py

@@ -9,14 +9,12 @@ from urllib.parse import urljoin, urlparse, parse_qs
 import httpx
 from bs4 import BeautifulSoup
 
-from src.constants import SEARXNG_INSTANCE
+from src.constants import SEARXNG_INSTANCE, REQUEST_TIMEOUT
 from .analytics import RateLimitError, error_logger
 from .query import build_enhanced_query
 
 logger = logging.getLogger(__name__)
 
-REQUEST_TIMEOUT = 20
-
 # Provider registry — maps setting value to (label, needs_key, needs_url)
 PROVIDER_INFO = {
     "searxng":  ("SearXNG",           False, True),