Feat/add drop support

[mtthidoteu]

Implements comprehensive Drop media server integration following the media-server integration checklist:

Core Implementation

• Create DropClient with RestApiMixin base class
• Implement full user management API (CRUD operations)
• Support Drop's two-step user creation (invitation → signup)
• Bearer token authentication with System tokens
• Proper error handling for missing Simple Authentication

Integration Points

• Register client via @register_media_client decorator
• Add connection checker with API validation
• Wire into media_servers routes and forms
• Update UI templates (create/edit server modals)

API Features

• User listing and synchronization from Drop API
• Statistics and server health monitoring
• Invitation-based user creation workflow
• Proper policy mapping (enabled, admin, displayName)

Error Handling

• Clear messaging when Simple Authentication disabled
• Token permission validation and logging
• Graceful fallback for missing API endpoints

Drop integration ready for use once Simple Authentication is enabled on the Drop server.

[mtthidoteu]

@DecDuck I have most of the things ready. However I am getting a 404 on /api/v1/admin/invitation. I tried manually doing it:

➜  ~   curl -X POST http://192.168.10.5:3010/api/v1/admin/invitation \
>     -H "Authorization: Bearer {TOKEN HERE}" \
>     -H "Content-Type: application/json" \
>     -d '{
quote>       "expires": "2025-09-12T14:00:00.000Z",
quote>       "username": "testuser",
quote>       "email": "[email protected]", 
quote>       "isAdmin": false
quote>     }'
{
  "error": true,
  "url": "http://192.168.10.5:3010/api/v1/admin/invitation",
  "statusCode": 404,
  "statusMessage": "Page not found: /api/v1/admin/invitation",
  "message": "Page not found: /api/v1/admin/invitation",
  "data": {
    "path": "/api/v1/admin/invitation"
  }
}%    

as per https://developer.droposs.org/web/users#create-invitation