Feature: Admin dashboard & moderation tools

[akargi]

Add an Admin Dashboard module to view key data, moderate listings, and manage users.

Direction:

• Build Admin pages with user list, listing moderation queue, and activity feed.
• Provide actions for banning users, removing listings, and resolving disputes.
• Protect admin routes with role-based access control.

Acceptance criteria:

• Admin dashboard displays counts and lists for users and pending moderation items.
• Moderation actions perform expected changes and are audited in logs.
• Admin routes require appropriate roles; non-admins cannot access them.