[SandObserver]

Summary

• Remove the errors key from the log threshold table, leaving the four level names the rank table already defines.
• Add a test pinning the accepted names, and one pinning that warnings still emit at the error level.

The removed name was documented nowhere, offered by no UI, and rejected by the settings route, which validates the stored value against debug, info and error. It was reachable only by setting LOG_LEVEL=errors by hand.

error deliberately keeps the same threshold as warn: the Settings tip reads "Errors shows warnings and errors only", so raising it would silently drop warnings for every install that selected it. The new test pins that.

Testing
All green, including the Docker build. Added 2 tests; the documented level list is derived from the rank table, so it was unaffected.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed. No documentation referenced the removed name.
• CHANGELOG.md updated under Unreleased if the change is user-visible. Not user-visible: the name was unreachable through any documented or offered path.