feat(application): audit abstractions (UDT-010 B2)

Introduces the contract layer for audit logging per design #D-8:

SIGCM2.Application/Audit/:
- IAuditContext — request-scoped accessor for ActorUserId/ActorRoleId/
  Ip/UserAgent/CorrelationId. Populated by CorrelationIdMiddleware +
  AuditActorMiddleware (B4).
- IAuditLogger.LogAsync(action, targetType, targetId, metadata?, ct) —
  domain-level audit emitter. Enlists in ambient TransactionScope
  (fail-closed per #REQ-AUD-4).
- ISecurityEventLogger.LogAsync(action, result, actorUserId?, attemptedUsername?,
  sessionId?, failureReason?, metadata?, ct) — security-events emitter
  separate from IAuditLogger (different retention, no transaction scope,
  captures login/logout/refresh/permission.denied).
- AuditOptions — bindable POCO with SanitizedKeys[] defaults (used by
  JsonSanitizer in B3).
- AuditEventDto — read projection for GET /api/v1/audit/events (B10).
- AuditEventFilter — query filter record with default Limit=50.

SIGCM2.Domain/Exceptions/:
- AuditContextMissingException : DomainException — fail-closed sentinel
  thrown when IAuditLogger is called without ActorUserId in a user-scoped
  command (#REQ-AUD-4).

Tests (Strict TDD — shape contract, RED → GREEN):
- tests/SIGCM2.Application.Tests/Audit/AuditAbstractionsTests.cs: 11 tests
  covering nullability, signatures, default options, record equality.

Suite: 336/336 Application.Tests (prev 325 + 11 new). 130/130 Api.Tests.

Refs: sdd/udt-010-auditoria-trazabilidad/{spec#REQ-AUD-3/4/5, design#D-8, tasks#B2}

src/api/SIGCM2.Application/Audit/AuditEventDto.cs

@@ -0,0 +1,15 @@
+namespace SIGCM2.Application.Audit;
+
+/// Read projection of dbo.AuditEvent for GET /api/v1/audit/events.
+/// ActorUsername is resolved by join against dbo.Usuario at query time (denormalized in the DTO).
+public sealed record AuditEventDto(
+    long Id,
+    DateTime OccurredAt,
+    int? ActorUserId,
+    string? ActorUsername,
+    string Action,
+    string TargetType,
+    string TargetId,
+    Guid? CorrelationId,
+    string? IpAddress,
+    string? Metadata);