[M2][Epic 2.4] Implement report generation and export (JSON/CSV)

[POWDER-RANGER]

Problem

Allow users to generate and download reports. Start with structured formats before PDF.

Tasks

• Create reports table (report_type, parameters, status, file_path)
• Implement POST /api/reports (create report job)
• Implement report worker/queue (Bull/BullMQ recommended)
• Generate JSON report (all documents + analyses)
• Generate CSV report (flattened data)
• Implement GET /api/reports/{id} (status check)
• Implement GET /api/reports/{id}/download (file download)
• Add report history to dashboard
• (Optional for M3) Add PDF generation

Acceptance Criteria

• ✅ Report job created and tracked via API
• ✅ JSON and CSV downloads work
• ✅ Report generation doesn't block API requests
• ✅ (Future) Scheduled reports can be configured

Priority: P2 - Medium
Labels: backend, reports, M2, P2