docs: current BMA architecture

[MagnusKlink]

Documents the current BMA architecture and image pipeline.

Add current-state documentation covering:

• Overall application architecture and subsystem boundaries
• BaseFile, Image, ImageVersion, Thumbnail, and ThumbnailSource
• image jobs, external CLI processing, and storage
• Django views, templates, and frontend presentation
• Current dependency status and Django 6 assessment
• Mermaid diagrams of model relationships and processing flow

This documents the existing system only; it does not propose changes.

Having spent a substantial amount of time getting a grasp of the shape and size of the current BMA, I hope this documentation gives the next newcomer a much clearer starting point. 😁