Implement: Signalering Widgets Specification

[rubenvdlinde]

OpenSpec Change: signalering-widgets

Spec path: openspec/changes/signalering-widgets

Generated from Specter (1 features).

[rubenvdlinde]

Hydra Builder has created a draft PR for the signalering-widgets feature: #254

This PR implements the backend services, controllers, and notification infrastructure for deadline alerting. Phase 2 Vue components and integration with case workflows are tracked in the task list.