fix: add HuggingFace mirror support for China network

- Add HF_ENDPOINT configuration option
- Set HuggingFace mirror to https://hf-mirror.com
- Fix 'cannot connect to huggingface.co' error
- Update .env.example with HF_ENDPOINT setting

Problem:
- RAG engine uses sentence-transformers model
- Model download requires connection to huggingface.co
- China network cannot access huggingface.co
- Error: 'We couldn't connect to https://huggingface.co'

Solution:
- Add HF_ENDPOINT environment variable support
- Use hf-mirror.com as HuggingFace mirror
- Set mirror before loading sentence-transformers
- Document in .env.example

Files:
- backend/app/config.py: add HF_ENDPOINT config
- backend/app/core/rag_engine.py: set HF_ENDPOINT before model load
- backend/setup_hf_mirror.sh: setup script
- backend/.env: configure mirror (not tracked)

backend/setup_hf_mirror.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# 设置 HuggingFace 镜像环境变量
+
+echo "配置 HuggingFace 镜像..."
+
+# 添加到 .env 文件
+if ! grep -q "HF_ENDPOINT" /data/erp-ass/backend/.env; then
+    echo "" >> /data/erp-ass/backend/.env
+    echo "# HuggingFace Mirror" >> /data/erp-ass/backend/.env
+    echo "HF_ENDPOINT=https://hf-mirror.com" >> /data/erp-ass/backend/.env
+    echo "✓ 已添加 HF_ENDPOINT 到 .env"
+else
+    echo "✓ HF_ENDPOINT 已存在"
+fi
+
+# 设置当前会话环境变量
+export HF_ENDPOINT=https://hf-mirror.com
+
+echo ""
+echo "环境变量已设置:"
+echo "  HF_ENDPOINT=https://hf-mirror.com"
+echo ""
+echo "请重启后端服务:"
+echo "  ./stop.sh && ./start.sh"