Point SEARXNG_INSTANCE to standalone searxng at host.docker.internal:8888

Bundled searxng was removed — the old http://searxng:8080 DNS name
resolves to nothing. Standalone searxng runs on Helm at port 8888,
reachable from container via host.docker.internal.

docker-compose.yml

@@ -29,7 +29,7 @@ services:
       - RESEARCH_LLM_ENDPOINT=${RESEARCH_LLM_ENDPOINT:-}
       - HF_TOKEN=${HF_TOKEN:-}
       - HUGGING_FACE_HUB_TOKEN=${HUGGING_FACE_HUB_TOKEN:-}
-      - SEARXNG_INSTANCE=http://searxng:8080
+      - SEARXNG_INSTANCE=http://host.docker.internal:8888
       - CHROMADB_HOST=chromadb
       - CHROMADB_PORT=8000
       - DATABASE_URL=${DATABASE_URL:-sqlite:///./data/app.db}