fix: docker-compose 持久化 SQLite 数据库; 迁移 DB 路径到 data/ 子目录

- docker-compose.yml: bind mount → named volume (auv_data:/app/data)
- backend/database.py: DB_PATH 改为 data/stock_data.db，自动迁移旧数据库
- .gitignore: 忽略 *.db 文件

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

docker-compose.yml

@@ -7,3 +7,8 @@ services:
     env_file:
       - .env
     restart: unless-stopped
+    volumes:
+      - auv_data:/app/data
+
+volumes:
+  auv_data: