revert: disable Cross-Encoder reranker

Live verification via POST /status/models/ping (after confirming the
gateway itself is reachable -- embedding role succeeded, 1.5s latency)
shows http://6.86.80.4:30080/v1/rerank returns a fast, reproducible
'503 Service Unavailable' -- not a timeout/fluke. The gateway's model
catalog (19 models: deepseek-*, glm-*, kimi-*, qwen3*, text-embedding-v3/4)
contains no cross-encoder/rerank-capable model, confirming no reranker
service is deployed behind this gateway today. Leaving RERANKER_ENABLED=true
would be a permanent no-op (graceful fallback to unranked order every call)
plus a misleading permanent error badge on the Status page. Revert until a
reranker model is actually deployed on the gateway.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.env

@@ -102,7 +102,7 @@ DOCUMENT_PARSE_ARTIFACT_PREFIX=artifacts
 PARSER_FAILURE_MODE=fail
 
 # ===== Reranker 配置 =====
-RERANKER_ENABLED=true
+RERANKER_ENABLED=false
 RERANKER_BASE_URL=http://6.86.80.4:30080/v1
 RERANKER_MODEL=BAAI/bge-reranker-v2-m3
 RERANKER_API_KEY=sk-fVr9KmDZNC4pGDBQj0EUWz9bDmFzNxjYC9EzZpe2bVDsxtz8