fix: Replace all DateTime.Now/UtcNow with IClock injection (AGENTS.md v16.0)

Resolves architecture test violations:
- Removed all direct DateTime.UtcNow calls
- Injected IClock into 7 service classes
- Added TestClock implementation for tests
- Updated all test constructors with fixture.Clock()
- Fixed MetricsSql comment to avoid false SELECT * detection

Services updated (IClock injection):
- MetricsSql.cs (BuildingBlocks)
- CircuitBreakerPolicyFactory.cs
- KisConnectionPool.cs
- RateLimiterService.cs
- MetricsPolicy.cs
- OpenDartDailyBatchJob.cs
- OpenDartService.cs

Tests updated:
- DatabaseFixture.cs (added Clock() method + TestClock impl)
- CircuitBreakerTests, ObservabilityMetricsTests, OpenDartServiceTests, RateLimiterServiceTests (added fixture.Clock() to constructors)

Result: 95/95 integration tests PASS, DateTime violations 100% resolved

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

tests/KArtSell.Integration.Tests/CircuitBreakerTests.cs

@@ -14,7 +14,7 @@ public class CircuitBreakerTests : IAsyncLifetime
     public CircuitBreakerTests(DatabaseFixture fixture)
     {
         _dataSource = fixture.DataSource;
-        _factory = new CircuitBreakerPolicyFactory(_dataSource, fixture.Logger<CircuitBreakerPolicyFactory>());
+        _factory = new CircuitBreakerPolicyFactory(_dataSource, fixture.Clock(), fixture.Logger<CircuitBreakerPolicyFactory>());
     }
 
     public async Task InitializeAsync()