Adopt TimeProvider instead of DateTime.UtcNow for testable time handling #1601

The codebase reads DateTime.UtcNow / DateTimeOffset.UtcNow directly in several places, which makes time-dependent behavior (expiry, retention, timestamps) hard to test deterministically.

Proposal: register TimeProvider.System in DI, inject TimeProvider where time is read, and use FakeTimeProvider (Microsoft.Extensions.TimeProvider.Testing) in tests.

Surfaced while reviewing #1267 (reading time tracker).