Rename User model to Admin and web guard to admin guard

[smnmsr]

Context

Current auth setup uses App\\Models\\User for admin users and auth:web middleware for admin routes. This works but naming is confusing now that auth:external exists.

Goal

Rename admin auth surface to explicit admin naming:

• App\\Models\\User -> App\\Models\\Admin
• auth:web guard -> auth:admin
• Admin routes use auth:admin

Scope

• Add admin guard/provider in config/auth.php
• Migrate admin routes and tests from auth:web to auth:admin
• Update factories, notifications, type hints, policies, and references
• Keep compatibility strategy for transition (temporary alias or phased migration)

Acceptance Criteria

• Admin authentication works unchanged for users
• All admin routes protected by auth:admin
• External routes stay on auth:external
• Test suite green