[FE-35] Build /admin/documents page — platform-wide document table with risk and status filters

[mftee]

Overview

Administrators need to see all documents across the platform — not just their own. This is essential for oversight, detecting suspicious patterns, and resolving disputes that involve documents from different users.

Background

File to create: frontend/app/(protected)/admin/documents/page.tsx

Access: Admin role only

Features:

• Paginated document table fetched from GET /api/admin/documents (BE-71)
• Columns: Document name, Owner name/email, Status badge, Risk score (colour-coded), Uploaded date, Flags
• Filters: Status, Risk score range (slider), Date range, Owner (search by email)
• Click a row → navigates to the document's detail page (/documents/:id)
• "Download" button per row → calls GET /api/documents/:id/export/pdf

Acceptance Criteria

• Table fetches from the admin endpoint (not the user-scoped endpoint)
• Owner email visible per row
• Risk score colour coding: green/yellow/red
• All filters combinable — applied as query params
• Empty state shown when no documents match the filter criteria