Add CSV export for statistics and user data

[OlivierJM]

Feature Parity: CSV Data Export

Legacy app allows admins to export statistics and user data as CSV files; sparked-next has no export capability.

Legacy capabilities

• Export platform statistics to CSV
• Include user profile data in exports (name, email, role)
• Include usage metrics per user
• Downloadable directly from admin dashboard

Acceptance criteria

• GET /api/admin/export/statistics — generate and download CSV of usage statistics
• GET /api/admin/export/users — generate and download CSV of user data
• Exports include relevant metadata (timestamps, user info, content info)
• Export button on statistics dashboard page
• Proper CSV MIME type and Content-Disposition headers for browser download

Identified via feature-parity audit against legacy Sparked (Meteor) app.