Plugin scaffold and settings

[andrewjohnson26]

Title: Plugin scaffold & settings

Description
Create nodebb-plugin-media-upload scaffold. Add ACP settings page with:

• Enable/disable toggle
• Max file size setting (in MB)

Assignee: Andrew
Estimated Effort: 2h
Dependencies: —

Acceptance Criteria

• Plugin installs & activates without error.
• ACP settings page appears under Plugins.
• Enable/disable toggle persists after save.
• Max file size limit persists after save.