feat: global exception handler, concurrent VLM, and docker security

- Add global FastAPI exception handler for unhandled errors
- Add get_current_user_obj() dependency for cleaner auth patterns
- Switch VLM image description from serial to asyncio.gather concurrency
- Extract score conversion to shared score_utils module
- Docker: use env vars for passwords, remove hardcoded API key default
- Add .gitattributes and update .gitignore for tar.gz and tsbuildinfo

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

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     environment:
       POSTGRES_DB: course_agent_db
       POSTGRES_USER: user
-      POSTGRES_PASSWORD: password
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-password}
       POSTGRES_INITDB_ARGS: "--encoding=UTF8 --locale=C"
     volumes:
       - postgres_data:/var/lib/postgresql/data
@@ -35,7 +35,7 @@ services:
       - "8001:8001"  # 前端应用
     environment:
       # 数据库配置
-      DATABASE_URL: postgresql+psycopg://user:password@db:5432/course_agent_db
+      DATABASE_URL: postgresql+psycopg://user:${POSTGRES_PASSWORD:-password}@db:5432/course_agent_db
       
       # JWT配置
       SECRET_KEY: ${SECRET_KEY:-your-super-secret-key-change-in-production}
@@ -43,7 +43,7 @@ services:
       ACCESS_TOKEN_EXPIRE_MINUTES: 30
       
       # 硅基流动API配置
-      SILICONFLOW_API_KEY: ${SILICONFLOW_API_KEY:-sk-pvvtosiglncktlucwarxilvsypqcttqizgpcfdvodgcuaezn}
+      SILICONFLOW_API_KEY: ${SILICONFLOW_API_KEY:?请设置 SILICONFLOW_API_KEY 环境变量}
       SILICONFLOW_BASE_URL: ${SILICONFLOW_BASE_URL:-https://api.siliconflow.cn/v1}
       SILICONFLOW_MODEL: ${SILICONFLOW_MODEL:-Qwen/Qwen3-30B-A3B-Thinking-2507}