feat(api): audit context middleware + scoped impl (UDT-010 B4)

Wires the request-scoped audit context per design #D-2:

Middleware pipeline in Program.cs:
  app.UseCors()
  app.UseMiddleware<CorrelationIdMiddleware>()  // PRE-AUTH
  app.UseAuthentication()
  app.UseMiddleware<AuditActorMiddleware>()     // POST-AUTH
  app.UseAuthorization()
  app.MapControllers()

SIGCM2.Api/Middleware/CorrelationIdMiddleware.cs:
- Preserves client-sent X-Correlation-Id header when a valid GUID, otherwise
  generates Guid.NewGuid(). Stores in HttpContext.Items (audit:correlationId).
- Captures Ip (Connection.RemoteIpAddress) + UserAgent header into Items.
- Echoes the correlation id back via response header (OnStarting + immediate
  set — immediate set makes unit testing against DefaultHttpContext reliable).

SIGCM2.Api/Middleware/AuditActorMiddleware.cs:
- Reads JWT 'sub' claim from authenticated HttpContext.User, parses to int,
  stores as audit:actorUserId. Anonymous / non-numeric sub leaves it unset.

SIGCM2.Infrastructure/Audit/AuditContext.cs (IAuditContext scoped impl):
- Reads Items entries via IHttpContextAccessor. Returns null / Guid.Empty
  when no HttpContext is available (jobs, tests without middleware).
- ActorRoleId intentionally null for now — rol code → id resolution is
  deferred; the logger may resolve it at persist time in a later batch.

DI registration (Infrastructure/DependencyInjection.cs):
- services.AddScoped<IAuditContext, AuditContext>()

Tests (Strict TDD):
- CorrelationIdMiddlewareTests (6): generates/preserves/handles-malformed
  correlation id, sets response header, captures ip/ua, calls next.
- AuditActorMiddlewareTests (5): authenticated/anonymous/no-sub/non-numeric/
  calls-next.
- AuditContextTests (7): reads from Items, null-http-context defaults,
  ActorRoleId currently null.

Suite: 355/355 Application.Tests + 141/141 Api.Tests = 496/496 passing.

Refs: sdd/udt-010-auditoria-trazabilidad/{spec#REQ-AUD-3/9, design#D-2, tasks#B4}

tests/SIGCM2.Api.Tests/Audit/AuditActorMiddlewareTests.cs

@@ -0,0 +1,87 @@
+using System.Security.Claims;
+using FluentAssertions;
+using Microsoft.AspNetCore.Http;
+using SIGCM2.Api.Middleware;
+using Xunit;
+
+namespace SIGCM2.Api.Tests.Audit;
+
+/// UDT-010 Batch 4 — AuditActorMiddleware unit tests (Strict TDD).
+/// Reads ActorUserId from the JWT "sub" claim after auth middleware populates HttpContext.User.
+public sealed class AuditActorMiddlewareTests
+{
+    [Fact]
+    public async Task Invoke_AuthenticatedUserWithSubClaim_SetsActorUserId()
+    {
+        var ctx = new DefaultHttpContext();
+        ctx.User = new ClaimsPrincipal(new ClaimsIdentity(new[]
+        {
+            new Claim("sub", "42"),
+            new Claim("rol", "admin"),
+        }, authenticationType: "Bearer"));
+
+        var mw = new AuditActorMiddleware(_ => Task.CompletedTask);
+        await mw.InvokeAsync(ctx);
+
+        ctx.Items["audit:actorUserId"].Should().Be(42);
+    }
+
+    [Fact]
+    public async Task Invoke_AnonymousRequest_LeavesActorUserIdNull()
+    {
+        var ctx = new DefaultHttpContext();
+        // User is an unauthenticated ClaimsPrincipal by default
+        var mw = new AuditActorMiddleware(_ => Task.CompletedTask);
+
+        await mw.InvokeAsync(ctx);
+
+        ctx.Items.TryGetValue("audit:actorUserId", out var value).Should().BeFalse();
+        value.Should().BeNull();
+    }
+
+    [Fact]
+    public async Task Invoke_AuthenticatedWithoutSubClaim_LeavesActorUserIdNull()
+    {
+        var ctx = new DefaultHttpContext();
+        ctx.User = new ClaimsPrincipal(new ClaimsIdentity(new[]
+        {
+            new Claim("name", "admin"),
+        }, authenticationType: "Bearer"));
+
+        var mw = new AuditActorMiddleware(_ => Task.CompletedTask);
+        await mw.InvokeAsync(ctx);
+
+        ctx.Items.TryGetValue("audit:actorUserId", out _).Should().BeFalse();
+    }
+
+    [Fact]
+    public async Task Invoke_SubClaimIsNonNumeric_LeavesActorUserIdNull()
+    {
+        var ctx = new DefaultHttpContext();
+        ctx.User = new ClaimsPrincipal(new ClaimsIdentity(new[]
+        {
+            new Claim("sub", "not-an-int"),
+        }, authenticationType: "Bearer"));
+
+        var mw = new AuditActorMiddleware(_ => Task.CompletedTask);
+        await mw.InvokeAsync(ctx);
+
+        ctx.Items.TryGetValue("audit:actorUserId", out _).Should().BeFalse();
+    }
+
+    [Fact]
+    public async Task Invoke_CallsNextDelegate()
+    {
+        var ctx = new DefaultHttpContext();
+        var nextCalled = false;
+        var mw = new AuditActorMiddleware(_ =>
+        {
+            nextCalled = true;
+            return Task.CompletedTask;
+        });
+
+        await mw.InvokeAsync(ctx);
+
+        nextCalled.Should().BeTrue();
+    }
+}