chore: enable Cross-Encoder reranker

Sandbox verification via /status/models/ping was inconclusive: the gateway
(6.86.80.4:30080) is unreachable from this environment entirely (embedding
role failed with the identical connection-timeout pattern, which is a
feature that definitely works in real deployment) -- not evidence that
/rerank specifically is unsupported. Reranker code already has graceful
TEI/Cohere fallback + falls back to unranked order on any failure, so this
is zero-risk to retrieval even if misconfigured. Please verify via
POST /status/models/ping in an environment with real gateway access;
revert RERANKER_ENABLED to false if that shows a genuine error.

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=false
+RERANKER_ENABLED=true
 RERANKER_BASE_URL=http://6.86.80.4:30080/v1
 RERANKER_MODEL=BAAI/bge-reranker-v2-m3
 RERANKER_API_KEY=sk-fVr9KmDZNC4pGDBQj0EUWz9bDmFzNxjYC9EzZpe2bVDsxtz8