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>

docker/docker-compose.deps-only.yml

@@ -1,7 +1,7 @@
 # =============================================================================
 # SurfSense — Dependencies only (no backend / frontend / Celery images)
 # =============================================================================
-# Postgres, Redis, SearXNG, pgAdmin, Zero — run API + Next + Celery on the host.
+# Postgres, Redis, pgAdmin, Zero — run API + Next + Celery on the host.
 # Celery is not Dockerized here: use `uv run` from surfsense_backend/ (no extra
 # backend image build just for workers).
 #
@@ -9,7 +9,7 @@
 #   docker compose -f docker/docker-compose.deps-only.yml up -d
 #
 # Compose variable substitution uses `docker/.env` (copy from .env.example).
-# Bind mounts use ./postgresql.conf and ./searxng in this directory.
+# Bind mounts use ./postgresql.conf in this directory.
 #
 # Local Celery (from surfsense_backend/, after Redis is up):
 #   uv run celery -A celery_worker.celery_app worker --loglevel=info --concurrency=1 --pool=solo --queues=surfsense,surfsense.connectors
@@ -17,7 +17,6 @@
 #
 # Host setup:
 #   - Backend .env: DATABASE_URL=postgresql+asyncpg://postgres:postgres@localhost:5432/surfsense
-#   - Backend .env: SEARXNG_DEFAULT_HOST=http://localhost:${SEARXNG_PORT:-8888}
 #   - Backend .env: CELERY_BROKER_URL / REDIS_APP_URL → redis://localhost:6379/0
 #   - Web .env: NEXT_PUBLIC_ZERO_CACHE_URL=http://localhost:${ZERO_CACHE_PORT:-4848}
 #
@@ -80,20 +79,6 @@ services:
       timeout: 5s
       retries: 5
 
-  searxng:
-    image: searxng/searxng:2026.3.13-3c1f68c59
-    ports:
-      - "${SEARXNG_PORT:-8888}:8080"
-    volumes:
-      - ./searxng:/etc/searxng
-    environment:
-      - SEARXNG_SECRET=${SEARXNG_SECRET:-surfsense-searxng-secret}
-    healthcheck:
-      test: ["CMD", "wget", "--spider", "-q", "http://localhost:8080/healthz"]
-      interval: 10s
-      timeout: 5s
-      retries: 5
-
   # NOTE: zero-cache requires the `zero_publication` Postgres publication to
   # exist before it starts. In this deps-only stack there is no backend
   # container to run migrations, so you must run `uv run alembic upgrade head`