fix(scanner): Fix WebService test infrastructure failure

- Update PostgresIdempotencyKeyRepository to use ScannerDataSource instead
  of NpgsqlDataSource directly (aligns with other Postgres repositories)
- Move IIdempotencyKeyRepository registration from IdempotencyMiddlewareExtensions
  to ServiceCollectionExtensions.RegisterScannerStorageServices
- Use Dapper instead of raw NpgsqlCommand for consistency
- Fixes: System.InvalidOperationException: Unable to resolve service for type
  'Npgsql.NpgsqlDataSource' when running WebService tests

Sprint planning:
- Create SPRINT_3500_0004_0001 CLI Verbs & Offline Bundles
- Create SPRINT_3500_0004_0002 UI Components & Visualization
- Create SPRINT_3500_0004_0003 Integration Tests & Corpus
- Create SPRINT_3500_0004_0004 Documentation & Handoff

Sprint: SPRINT_3500_0002_0003

src/Scanner/StellaOps.Scanner.WebService/Middleware/IdempotencyMiddlewareExtensions.cs

@@ -8,8 +8,6 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
-using StellaOps.Scanner.Storage.Postgres;
-using StellaOps.Scanner.Storage.Repositories;
 using StellaOps.Scanner.WebService.Options;
 
 namespace StellaOps.Scanner.WebService.Middleware;
@@ -21,6 +19,7 @@ public static class IdempotencyMiddlewareExtensions
 {
     /// <summary>
     /// Adds idempotency services to the service collection.
+    /// Note: IIdempotencyKeyRepository is registered by AddScannerStorage.
     /// </summary>
     public static IServiceCollection AddIdempotency(
         this IServiceCollection services,
@@ -32,8 +31,6 @@ public static class IdempotencyMiddlewareExtensions
         services.Configure<IdempotencyOptions>(
             configuration.GetSection(IdempotencyOptions.SectionName));
 
-        services.AddScoped<IIdempotencyKeyRepository, PostgresIdempotencyKeyRepository>();
-
         return services;
     }