feat: add OpenSERP self-hosted web search backend (#27437)

Add self-hosted OpenSERP as a web search engine option. OpenSERP
provides browser-rendered search across Google, Bing, Yandex, Baidu,
DuckDuckGo, and Ecosia with no API keys required.

- New module: retrieval/web/openserp.py (async, uses aiohttp session pool)
- Config: OPENSERP_BASE_URL env var (defaults to http://localhost:7070)
- Routing: search_web() dispatch for 'openserp' engine
- Follows existing patterns (searxng, brave)

Co-authored-by: crustopher-lgtm <crustopher-lgtm@users.noreply.github.com>

backend/open_webui/config.py

@@ -1176,6 +1176,7 @@ WEB_SEARCH_TRUST_ENV = os.getenv('WEB_SEARCH_TRUST_ENV', 'True').lower() == 'tru
 OLLAMA_CLOUD_WEB_SEARCH_API_KEY = os.getenv('OLLAMA_CLOUD_API_KEY', '')
 
 SEARXNG_QUERY_URL = os.getenv('SEARXNG_QUERY_URL', '')
+OPENSERP_BASE_URL = os.getenv('OPENSERP_BASE_URL', 'http://localhost:7000')
 
 SEARXNG_LANGUAGE = os.getenv('SEARXNG_LANGUAGE', 'all')
 
@@ -2907,6 +2908,7 @@ DEFAULT_CONFIG = {
     'web.search.trust_env': WEB_SEARCH_TRUST_ENV,
     'web.search.ollama_cloud_api_key': OLLAMA_CLOUD_WEB_SEARCH_API_KEY,
     'web.search.searxng_query_url': SEARXNG_QUERY_URL,
+    'web.search.openserp_base_url': OPENSERP_BASE_URL,
     'web.search.searxng_language': SEARXNG_LANGUAGE,
     'web.search.yacy_query_url': YACY_QUERY_URL,
     'web.search.yacy_username': YACY_USERNAME,