feat(infra): audit + security event repositories (UDT-010 B5)

Introduces persistence layer for audit and security events per design #D-6:

SIGCM2.Application/Audit/:
- IAuditEventRepository: InsertAsync + QueryAsync with cursor pagination
- ISecurityEventRepository: InsertAsync only (no query — SecurityEvent is
  queried only from an admin dashboard deferred to ADM-004)
- AuditEventQueryResult: (Items, NextCursor) record

SIGCM2.Infrastructure/Audit/:
- AuditEventCursor (public): base64(OccurredAt:O|Id) opaque cursor for
  DESC pagination. TryDecode is fail-open — malformed cursor returns null
  and the query starts from the top.
- AuditEventRepository: Dapper INSERT via OUTPUT INSERTED.Id + dynamic
  WHERE composition with parameterized filters (zero SQL injection risk).
  LEFT JOIN to dbo.Usuario to populate ActorUsername in AuditEventDto.
  Pagination fetches Limit+1 rows to detect "more pages"; emits cursor
  from the Nth row when overflow observed.
- SecurityEventRepository: straight INSERT for login/logout/refresh/
  permission.denied events.

DI: AddScoped for both repos in AddInfrastructure.

Integration tests (Strict TDD): 13 total, all against SIGCM2_Test.
- AuditEventRepositoryTests (10): insert-roundtrip, filter-by-actor,
  filter-by-target, filter-by-date-range, cursor pagination across 3 pages
  (no overlap/no gap), malformed-cursor fail-open, LEFT JOIN Usuario
  populates username, cursor encode/decode roundtrip, cursor malformed
  variants.
- SecurityEventRepositoryTests (3): insert success, insert failure with
  null ActorUserId + AttemptedUsername, CK_SecurityEvent_Result rejection.

Suite: 368/368 Application.Tests + 141/141 Api.Tests = 509/509 passing.

Refs: sdd/udt-010-auditoria-trazabilidad/{spec#REQ-AUD-2,7 #REQ-SEC-1,
design#D-6, tasks#B5}

src/api/SIGCM2.Infrastructure/DependencyInjection.cs

@@ -72,6 +72,8 @@ public static class DependencyInjection
         // UDT-010: Audit options (SanitizedKeys blacklist) — overridable via appsettings "Audit".
         services.Configure<AuditOptions>(configuration.GetSection(AuditOptions.SectionName));
         services.AddScoped<IAuditContext, SIGCM2.Infrastructure.Audit.AuditContext>();
+        services.AddScoped<IAuditEventRepository, SIGCM2.Infrastructure.Audit.AuditEventRepository>();
+        services.AddScoped<ISecurityEventRepository, SIGCM2.Infrastructure.Audit.SecurityEventRepository>();
 
         // Dispatcher
         services.AddScoped<IDispatcher, Dispatcher>();