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/.env.example

@@ -55,18 +55,6 @@ WHATSAPP_WEBHOOK_VERIFY_TOKEN=
 WHATSAPP_WEBHOOK_APP_SECRET=
 WHATSAPP_BRIDGE_URL=http://whatsapp-bridge:9929
 
-# Platform Web Search (SearXNG)
-# Set this to enable built-in web search. Docker Compose sets it automatically.
-# Only uncomment if running the backend outside Docker (e.g. uvicorn on host).
-# SEARXNG_DEFAULT_HOST=http://localhost:8888
-
-# web.discover fallback providers (env-keyed). SearXNG above is preferred; these
-# are used when it is not configured. web.discover self-disables if none are set.
-# LINKUP_API_KEY=
-# BAIDU_API_KEY=
-# BAIDU_MODEL=ernie-3.5-8k
-# BAIDU_SEARCH_SOURCE=baidu_search_v2
-
 # Periodic task interval
 # # Run every minute (default)
 # SCHEDULE_CHECKER_INTERVAL=1m