feat: file export design

[earayu]

Note

Adds a new Chinese design spec docs/design/document_export_design_zh.md describing the document export module.

• Defines architecture: backend-proxied downloads, single-document streaming vs. collection async export, FastAPI StreamingResponse, Celery+Redis, object storage paths
• Specifies API contracts and flows (/documents/{id}/download, /collections/{id}/export, /export-tasks/.../download) and ZIP packaging lifecycle
• Introduces export_task table schema, status transitions, progress tracking, limits/timeouts, and 7-day cleanup
• Details security, permissions, rate limiting, monitoring/auditing, and error handling
• Cross-references planned code locations: aperag/views/export.py, aperag/service/export_service.py, aperag/tasks/export.py, and object store drivers

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