Edge-label captions were using an ambient light-dark() background that
didn't match Gitea's actual dark theme. Fixed to a plain rgb(13,17,23),
matching the corrected CLAUDE.md diagram convention.
No more Mermaid, per standing rule - the architecture diagram is now
a real image (assets/architecture-overview.svg), built and exported
from the Nextcloud Diagramming app, fetched via WebDAV and validated
as well-formed XML before committing. Ends the rendering-gap chase
entirely since there's no live renderer involved anymore, just a
static image.