fix(config): config.example.py 补充 RERANK 云服务配置项

补充 RERANK_BACKEND/CLOUD_MODEL/CLOUD_API_KEY/CLOUD_BASE_URL/
CLOUD_TIMEOUT 五个配置项，防止新部署时 engine.py 初始化失败。

config.example.py

@@ -109,6 +109,11 @@ USE_RERANK = True
 RERANK_CANDIDATES = 20           # 送入重排序的候选数
 RERANK_TOP_K = 15                # 重排序后保留数
 RERANK_USE_ONNX = os.getenv("RERANK_USE_ONNX", "true").lower() == "true"
+RERANK_BACKEND = os.getenv("RERANK_BACKEND", "local")       # local / cloud / fallback
+RERANK_CLOUD_MODEL = os.getenv("RERANK_CLOUD_MODEL", "qwen3-rerank")
+RERANK_CLOUD_API_KEY = os.getenv("RERANK_CLOUD_API_KEY", "")
+RERANK_CLOUD_BASE_URL = os.getenv("RERANK_CLOUD_BASE_URL", "https://dashscope.aliyuncs.com/compatible-api/v1/reranks")
+RERANK_CLOUD_TIMEOUT = int(os.getenv("RERANK_CLOUD_TIMEOUT", "15"))
 RERANK_CONTEXT_MIN_SCORE = 0.05  # Rerank 分数低于此值的切片不送入 LLM
 
 # ----- RRF 融合 -----