Add configurable logo in top-left title area across web projects

[thusser]

Summary

Add an option to inject a logo into the title element in the upper left, across all pyobs web projects. Default should be the pyobs logo.

Notes

• Should be configurable (so deployments can override with their own logo).
• Should apply consistently across all web projects.

[thusser]

Implemented in pyobs-pipeline, pyobs-archive, pyobs-web-admin, and pyobs-robotic-backend: the pyobs wordmark now shows next to the sidebar header, with light/dark variants swapped automatically via the theme toggle from #749.

Configurable per deployment via PYOBS_LOGO_LIGHT_URL/PYOBS_LOGO_DARK_URL settings; defaults to the bundled logo.

pyobs-web-client not included (deferred along with its theme-switcher work, see #749's closing comment).