Fix: cap the widget failure log per widget

[SandObserver]

Summary

• Keep at most 16 failing keys per widget in the failure log. A new key past the cap is neither stored nor logged.
• Pass the widget id and the existing per-widget cap from GET /api/widget-data/:id, whose key includes the request's endpoint value.
• Change failed() to take { now, group, maxPerGroup } as its last argument.

Testing
All green. Added tests for the per-widget cap and for 40 distinct endpoint values producing 16 lines.

Changelog

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.