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.Host/Observability/OpenDartService.cs

@@ -1,4 +1,5 @@
 using Dapper;
+using KArtSell.BuildingBlocks.Time;
 using Microsoft.AspNetCore.Authentication;
 using Npgsql;
 using Microsoft.Extensions.Logging;
@@ -13,6 +14,7 @@ public class OpenDartService
 {
     private readonly NpgsqlDataSource _dataSource;
     private readonly HttpClient _httpClient;
+    private readonly IClock _clock;
     private readonly string _apiKey;
     private readonly ILogger<OpenDartService> _logger;
 
@@ -35,10 +37,12 @@ public class OpenDartService
     public OpenDartService(
         NpgsqlDataSource dataSource,
         HttpClient httpClient,
+        IClock clock,
         ILogger<OpenDartService> logger)
     {
         _dataSource = dataSource;
         _httpClient = httpClient;
+        _clock = clock;
         _apiKey = Environment.GetEnvironmentVariable("OPENDART_API_KEY") ?? throw new InvalidOperationException("OPENDART_API_KEY required");
         _logger = logger;
     }
@@ -89,7 +93,7 @@ public class OpenDartService
 #pragma warning disable DAP005
         var json = await connection.QueryFirstOrDefaultAsync<string>(
             sql,
-            new { ticker, quarter = quarterKey, now = DateTime.UtcNow },
+            new { ticker, quarter = quarterKey, now = _clock.UtcNow.UtcDateTime },
             commandTimeout: 5);
 
         if (json == null) return null;
@@ -137,7 +141,7 @@ public class OpenDartService
             """;
 
         var dataJson = System.Text.Json.JsonSerializer.Serialize(data);
-        var now = DateTime.UtcNow;
+        var now = _clock.UtcNow.UtcDateTime;
         var expiresAt = now.AddDays(CacheTtlDays);
 
         await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);