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);
+    }
+}

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

@@ -0,0 +1,51 @@
+using Microsoft.AspNetCore.Http;
+
+namespace SIGCM2.Api.Middleware;
+
+/// UDT-010 — pre-auth middleware that stamps every request with a correlation ID,
+/// preserves one sent by the client via X-Correlation-Id, and exposes it on the response.
+/// Also captures Ip + UserAgent for downstream IAuditContext consumers.
+public sealed class CorrelationIdMiddleware
+{
+    public const string HeaderName = "X-Correlation-Id";
+    public const string ItemCorrelationId = "audit:correlationId";
+    public const string ItemIp = "audit:ip";
+    public const string ItemUserAgent = "audit:userAgent";
+
+    private readonly RequestDelegate _next;
+
+    public CorrelationIdMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    public async Task InvokeAsync(HttpContext ctx)
+    {
+        Guid correlationId;
+        if (ctx.Request.Headers.TryGetValue(HeaderName, out var incoming)
+            && Guid.TryParse(incoming.ToString(), out var parsed))
+        {
+            correlationId = parsed;
+        }
+        else
+        {
+            correlationId = Guid.NewGuid();
+        }
+
+        ctx.Items[ItemCorrelationId] = correlationId;
+        ctx.Items[ItemIp] = ctx.Connection.RemoteIpAddress?.ToString();
+        ctx.Items[ItemUserAgent] = ctx.Request.Headers.UserAgent.ToString();
+
+        ctx.Response.OnStarting(() =>
+        {
+            ctx.Response.Headers[HeaderName] = correlationId.ToString("D");
+            return Task.CompletedTask;
+        });
+
+        // Also set immediately for testability — DefaultHttpContext does not trigger OnStarting
+        // in unit tests because no body is written through the pipeline.
+        ctx.Response.Headers[HeaderName] = correlationId.ToString("D");
+
+        await _next(ctx);
+    }
+}

src/api/SIGCM2.Api/Program.cs

@@ -2,6 +2,7 @@ using Microsoft.AspNetCore.Authorization;
 using Serilog;
 using Scalar.AspNetCore;
 using SIGCM2.Api.Authorization;
+using SIGCM2.Api.Middleware;
 using SIGCM2.Application;
 using SIGCM2.Infrastructure;
 using SIGCM2.Api.Filters;
@@ -66,7 +67,12 @@ if (app.Environment.IsDevelopment() || app.Environment.IsEnvironment("Testing"))
 
 app.UseHttpsRedirection();
 app.UseCors();
+// UDT-010: correlation id + ip/ua capture runs BEFORE auth so anonymous requests
+// still get a correlation id and so logs can tie pre-auth events to the request.
+app.UseMiddleware<CorrelationIdMiddleware>();
 app.UseAuthentication();
+// UDT-010: actor extraction runs AFTER auth to read the JWT sub claim.
+app.UseMiddleware<AuditActorMiddleware>();
 app.UseAuthorization();
 app.MapControllers();