feat: add DeepSeek official API support and capture reasoning content

- Add DeepSeek provider routing: deepseek-chat/deepseek-reasoner use
  api.deepseek.com, other models use SiliconFlow
- Add stream_with_reasoning() using raw OpenAI SDK to capture
  reasoning_content (langchain_openai strips this field)
- RAG chain and conversation chain both use stream_with_reasoning
  for proper reasoning display in thinking models
- Frontend model selector: grouped by provider (DeepSeek official +
  SiliconFlow), default changed to deepseek-chat
- Regenerate message converted to streaming with reasoning capture
- Minor UI: globals.css additions, chat store refactoring

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

env.example

@@ -3,6 +3,10 @@ SILICONFLOW_API_KEY=your-api-key-here
 SILICONFLOW_BASE_URL=https://api.siliconflow.cn/v1
 SILICONFLOW_MODEL=Qwen/Qwen3-30B-A3B-Thinking-2507
 
+# DeepSeek官方API配置（可选，不配则走硅基流动）
+# DEEPSEEK_API_KEY=your-deepseek-api-key-here
+DEEPSEEK_BASE_URL=https://api.deepseek.com
+
 # 数据库配置（本地开发默认使用SQLite）
 DATABASE_URL=sqlite:///../data/database/course_agent.db
 # Docker部署使用PostgreSQL