chore: 初始化项目与后端基础工程

backend/.env.example

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+APP_NAME="财务 AI 助手 API"
+APP_VERSION=1.0.0
+DEBUG=true
+SECRET_KEY=change-me-in-production
+ALLOWED_ORIGINS=http://localhost:3000
+
+DATABASE_URL=postgresql+asyncpg://s2f_user:s2f_password@localhost:5432/s2f_db
+
+JWT_SECRET_KEY=change-me-in-production
+JWT_ALGORITHM=HS256
+JWT_ACCESS_TOKEN_EXPIRE_MINUTES=60
+
+OPENAI_API_KEY=
+OPENAI_MODEL=gpt-4-turbo-preview
+ZHIPU_API_KEY=
+AI_PROVIDER=zhipu
+AI_API_KEY=
+
+UPLOAD_DIR=./uploads
+MAX_UPLOAD_SIZE=10485760
+
+LOG_LEVEL=INFO