add explain module

sentence_api/.env.example

@@ -17,6 +17,13 @@ MOSS_MAX_NEW_TOKENS=65536
 # 每句结束时间向后顺延的毫秒数，避免句子末尾发音被切掉；按需在 200-500 之间调整
 MOSS_END_PADDING_MS=300
 
+# AI sentence explanation (OpenAI-compatible chat completions endpoint).
+# The URL and key stay on the server; mobile clients only send the sentence text.
+AI_API_URL=https://api.example.com/v1/chat/completions
+AI_API_KEY=replace-with-your-ai-api-key
+AI_MODEL=gpt-4o-mini
+AI_TIMEOUT_SECONDS=45
+
 # 火山引擎对象存储（TOS / 火山 OSS）。桶请设置为公共读，或把下方地址配置为可公开访问的 CDN 域名。
 VOLCANO_OSS_ACCESS_KEY=replace-with-volcano-access-key
 VOLCANO_OSS_SECRET_KEY=replace-with-volcano-secret-key