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}

src/api/SIGCM2.Api/Middleware/AuditActorMiddleware.cs

@@ -0,0 +1,32 @@
+using Microsoft.AspNetCore.Http;
+
+namespace SIGCM2.Api.Middleware;
+
+/// UDT-010 — post-auth middleware that reads the JWT "sub" claim and stores the
+/// resolved ActorUserId in HttpContext.Items. Anonymous requests leave it unset.
+/// ActorRoleId is reserved for a future batch (rol code → id resolution).
+public sealed class AuditActorMiddleware
+{
+    public const string ItemActorUserId = "audit:actorUserId";
+
+    private readonly RequestDelegate _next;
+
+    public AuditActorMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    public async Task InvokeAsync(HttpContext ctx)
+    {
+        if (ctx.User.Identity?.IsAuthenticated == true)
+        {
+            var sub = ctx.User.FindFirst("sub")?.Value;
+            if (int.TryParse(sub, out var userId))
+            {
+                ctx.Items[ItemActorUserId] = userId;
+            }
+        }
+
+        await _next(ctx);
+    }
+}