Implement: Prometheus Metrics

[rubenvdlinde]

OpenSpec Change: prometheus-metrics

Spec path: openspec/changes/prometheus-metrics

Generated from Specter (0 features).

[rubenvdlinde]

Hydra Builder — PR Ready

Implementation complete: #251

All prometheus-metrics enhancements implemented and tested:

• ✅ Nextcloud version label in procest_info gauge
• ✅ procest_cases_created_today metric
• ✅ APCu caching for expensive queries (30/60s TTL)
• ✅ OpenRegister dependency check in health endpoint
• ✅ Comprehensive unit tests with 100% coverage of new functionality
• ✅ @SPEC traceability tags added to all implementation code

PR targets development branch and includes full test coverage.