Added instrastructure extensions

Implemented database service registration

PostFundManagement.Infrastructure/Extensions/Constants.cs

@@ -0,0 +1,6 @@
+namespace PostFundManagement.Infrastructure.Extensions;
+
+public static class Constants
+{
+    public const string DatabaseConfigName = "PfmDatabase";
+}

PostFundManagement.Infrastructure/Extensions/Postgres.cs

@@ -0,0 +1,17 @@
+using PostFundManagement.Infrastructure.Database;
+using static PostFundManagement.Infrastructure.Extensions.Constants;
+
+namespace PostFundManagement.Infrastructure.Extensions;
+
+public static class Postgres
+{
+    public static IServiceCollection AddApplicationDbContext(this IServiceCollection services, IConfiguration configuration)
+    {
+        var connectionString = configuration.GetConnectionString(DatabaseConfigName)
+            ?? throw new InvalidOperationException($"Connection string '{DatabaseConfigName}' was not found in configuration.");
+
+        services.AddPooledDbContextFactory<ApplicationDbContext>(builder => builder.UseNpgsql(connectionString));
+
+        return services;
+    }
+}