feat(agents): consolidate connectors under mcp_discovery; route web search through google_search

MCP consolidation:
- Route all MCP-capable connectors (Slack, Jira, Linear, ClickUp, Airtable,
  Notion, Confluence, interim Gmail/Calendar, custom MCP) through a single
  `mcp_discovery` subagent. Drive/OneDrive/Dropbox stay native to enrich the KB.
- Deprecate Discord/Teams/Luma: no viable official MCP server.

Google-only web search:
- Remove the main-agent `web_search` tool and the SearXNG platform service;
  all public web search now flows through the `google_search` subagent via task().
- Deprecate the Tavily/SearXNG/Linkup/Baidu search connectors (HTTP 410 on
  create, "Deprecated" badge); guide heavy users to the custom MCP connector.
- Remove web search from anonymous chat (pure Q&A).
- Tear SearXNG out of docker compose + install scripts; drop tavily-python
  and linkup-sdk deps and their config/env vars.

Fix:
- metrics._package_version() now swallows any metadata lookup failure. A
  malformed editable-install distribution with no `Version` field raised
  KeyError deep in importlib.metadata, and since it runs on every
  record_subagent_invoke_duration call it was crashing every task()
  delegation. Verified end-to-end against live GPT-5.4.

Co-authored-by: Cursor <cursoragent@cursor.com>

surfsense_backend/app/config/__init__.py

@@ -554,13 +554,6 @@ class Config:
         os.getenv("SURFSENSE_CONNECTOR_DISCOVERY_TTL_SECONDS", "30")
     )
 
-    # Platform web search providers for the web.discover capability (env-keyed).
-    SEARXNG_DEFAULT_HOST = os.getenv("SEARXNG_DEFAULT_HOST")
-    LINKUP_API_KEY = os.getenv("LINKUP_API_KEY")
-    BAIDU_API_KEY = os.getenv("BAIDU_API_KEY")
-    BAIDU_MODEL = os.getenv("BAIDU_MODEL", "ernie-3.5-8k")
-    BAIDU_SEARCH_SOURCE = os.getenv("BAIDU_SEARCH_SOURCE", "baidu_search_v2")
-
     SURFSENSE_PUBLIC_URL = os.getenv("SURFSENSE_PUBLIC_URL")
     NEXT_FRONTEND_URL = os.getenv("NEXT_FRONTEND_URL") or SURFSENSE_PUBLIC_URL
     # Backend URL to override the http to https in the OAuth redirect URI