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/Infrastructure/RateLimiterService.cs

@@ -1,4 +1,5 @@
 using Dapper;
+using KArtSell.BuildingBlocks.Time;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Logging;
 using Npgsql;
@@ -13,6 +14,7 @@ namespace KArtSell.Host.Infrastructure;
 public class RateLimiterService
 {
     private readonly NpgsqlDataSource _dataSource;
+    private readonly IClock _clock;
     private readonly ILogger<RateLimiterService> _logger;
 
     private static readonly Dictionary<string, RateLimitConfig> ApiConfigs = new()
@@ -34,9 +36,10 @@ public class RateLimiterService
             new EventId(2, nameof(LogQuotaExceeded)),
             "Rate limit: {ApiName} quota exceeded, retry after {RetryAfter}s");
 
-    public RateLimiterService(NpgsqlDataSource dataSource, ILogger<RateLimiterService> logger)
+    public RateLimiterService(NpgsqlDataSource dataSource, IClock clock, ILogger<RateLimiterService> logger)
     {
         _dataSource = dataSource;
+        _clock = clock;
         _logger = logger;
     }
 
@@ -66,7 +69,7 @@ public class RateLimiterService
         await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);
         var result = await connection.QueryFirstOrDefaultAsync<(int CurrentTokens, int WindowSeconds, int LimitCount)?>(
             sql,
-            new { apiName = apiName.ToLower(), now = DateTime.UtcNow },
+            new { apiName = apiName.ToLower(), now = _clock.UtcNow.UtcDateTime },
             commandTimeout: 5);
 
         if (result == null)
@@ -106,7 +109,7 @@ public class RateLimiterService
         await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);
         await connection.ExecuteAsync(
             sql,
-            new { apiName = apiName.ToLower(), limit = config.Limit, now = DateTime.UtcNow },
+            new { apiName = apiName.ToLower(), limit = config.Limit, now = _clock.UtcNow.UtcDateTime },
             commandTimeout: 5);
 
         _logger.LogInformation("Rate limit quota reset for {ApiName}: {Limit}/{WindowSeconds}s", apiName, config.Limit, config.WindowSeconds);
@@ -129,7 +132,7 @@ public class RateLimiterService
         {
             await connection.ExecuteAsync(
                 sql,
-                new { apiName, limit = config.Limit, window = config.WindowSeconds, now = DateTime.UtcNow },
+                new { apiName, limit = config.Limit, window = config.WindowSeconds, now = _clock.UtcNow.UtcDateTime },
                 commandTimeout: 5);
         }
 
@@ -148,7 +151,7 @@ public class RateLimiterService
             await using var connection = await _dataSource.OpenConnectionAsync(cancellationToken);
             await connection.ExecuteAsync(
                 sql,
-                new { apiName = apiName.ToLower(), action, now = DateTime.UtcNow },
+                new { apiName = apiName.ToLower(), action, now = _clock.UtcNow.UtcDateTime },
                 commandTimeout: 5);
         }
         catch (Exception ex)