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>

src/KArtSell.BuildingBlocks/Observability/MetricsSql.cs

@@ -1,4 +1,5 @@
 using Dapper;
+using KArtSell.BuildingBlocks.Time;
 using Npgsql;
 
 namespace KArtSell.BuildingBlocks.Observability;
@@ -6,15 +7,17 @@ namespace KArtSell.BuildingBlocks.Observability;
 /// <summary>
 /// Shared queries for observability metrics across all modules.
 /// All queries use PIT (point-in-time) pattern: published_at <= cutoff.
-/// Schema-qualified, explicit columns, no SELECT *.
+/// Schema-qualified, explicit columns, no wildcard column selection.
 /// </summary>
 public class MetricsSql
 {
     private readonly NpgsqlDataSource _dataSource;
+    private readonly IClock _clock;
 
-    public MetricsSql(NpgsqlDataSource dataSource)
+    public MetricsSql(NpgsqlDataSource dataSource, IClock clock)
     {
         _dataSource = dataSource;
+        _clock = clock;
     }
 
     public async Task<(int Total, int OnTime, TimeSpan AvgTime)?> GetBatchSlaAsync(CancellationToken cancellationToken = default)
@@ -29,10 +32,11 @@ public class MetricsSql
               AND completed_at >= @sevenDaysAgo
             """;
 
+        var now = _clock.UtcNow.UtcDateTime;
         await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);
         var result = await connection.QueryFirstOrDefaultAsync<(int, int, double)?>(
             sql,
-            new { now = DateTime.UtcNow, sevenDaysAgo = DateTime.UtcNow.AddDays(-7) },
+            new { now, sevenDaysAgo = now.AddDays(-7) },
             commandTimeout: 5);
 
         if (result == null || result.Value.Item1 == 0)
@@ -54,10 +58,11 @@ public class MetricsSql
               AND quarantined_at >= @sevenDaysAgo
             """;
 
+        var now = _clock.UtcNow.UtcDateTime;
         await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);
         var result = await connection.QueryFirstOrDefaultAsync<(int Total, int Quarantined, string? Errors)?>(
             sql,
-            new { now = DateTime.UtcNow, sevenDaysAgo = DateTime.UtcNow.AddDays(-7) },
+            new { now, sevenDaysAgo = now.AddDays(-7) },
             commandTimeout: 5);
 
         if (result == null || result.Value.Item1 == 0)