Add admin audit log for tracking admin actions

[darshacharya]

Description

There is no record of admin actions. When multiple admins are involved, it's important to track who approved/rejected which case and when.

Requirements

• Create an `AuditLog` model to record admin actions
• Log: case verification (approve/reject), document verification, any future admin actions
• Store: admin user ID, action type, target (case/document ID), timestamp, details
• Add `GET /api/admin/audit-log` endpoint (admin-only) with pagination
• Add an audit log page in the admin panel

Tech Notes

• New model: `backend/app/models/audit_log.py`
• Hook into `backend/app/routers/admin.py` — `verify_case` and `verify_document`
• Frontend: New page at `frontend/src/app/admin/audit/page.tsx`

Acceptance Criteria

• All admin actions are logged
• Audit log records who, what, when, and details
• Admin can view the audit log
• Log is paginated
• Audit entries cannot be modified or deleted