feat(alarmas): record and clear per-alarm scheduling failures

Adds ServicioAlarmas.registrarFalloProgramacion/limpiarFalloProgramacion,
persisting a scheduling-reliability failure through the same
ExcepcionAlarma model saltarProxima already uses. Only one failure
record is kept per alarm (latest attempt wins) and skipNext entries
for any alarm are never touched. EstadoAlarmas wiring follows next.

lib/servicios/servicio_alarmas.dart

@@ -350,6 +350,62 @@ class ServicioAlarmas {
     return nuevo;
   });
 
+  /// Records a scheduling-reliability failure for [alarmaId] (fix/alarmas-
+  /// fallos-silenciosos): reuses the SAME `ExcepcionAlarma` model
+  /// `saltarProxima` already persists, so `EstadoAlarmas.ultimaExcepcionPara`
+  /// surfaces it on the exact alarm card affected instead of only a
+  /// transient, alarm-agnostic message. A previous FAILURE record for the
+  /// same alarm is replaced (only the latest attempt's outcome matters) --
+  /// any `skipNext` exception for this or other alarms is left untouched.
+  /// Never affects scheduling: `ServicioProgramacionAlarmas._esValida` only
+  /// treats `tipoSaltoSiguiente` as an actual skip.
+  Future<ConfiguracionAlarmas> registrarFalloProgramacion(
+    String alarmaId,
+    DateTime ejecucion,
+    String tipo,
+  ) => _enCola(() async {
+    final config = await _configActual();
+    final excepciones = [
+      ..._sinFalloPrevio(config.excepciones, alarmaId),
+      ExcepcionAlarma(alarmaId: alarmaId, ejecucion: ejecucion, tipo: tipo),
+    ];
+    final nuevo = ConfiguracionAlarmas(
+      alarmas: config.alarmas,
+      vacaciones: config.vacaciones,
+      excepciones: excepciones,
+    );
+    await _guardar(nuevo);
+    return nuevo;
+  });
+
+  /// Clears any outstanding failure record for [alarmaId] (a subsequent
+  /// scheduling attempt succeeded). No-op when there is nothing to clear.
+  Future<ConfiguracionAlarmas> limpiarFalloProgramacion(
+    String alarmaId,
+  ) => _enCola(() async {
+    final config = await _configActual();
+    final sinFallo = _sinFalloPrevio(config.excepciones, alarmaId);
+    if (sinFallo.length == config.excepciones.length) return config;
+    final nuevo = ConfiguracionAlarmas(
+      alarmas: config.alarmas,
+      vacaciones: config.vacaciones,
+      excepciones: sinFallo,
+    );
+    await _guardar(nuevo);
+    return nuevo;
+  });
+
+  List<ExcepcionAlarma> _sinFalloPrevio(
+    List<ExcepcionAlarma> excepciones,
+    String alarmaId,
+  ) => excepciones
+      .where(
+        (e) =>
+            !(e.alarmaId == alarmaId &&
+                ExcepcionAlarma.tiposFallo.contains(e.tipo)),
+      )
+      .toList();
+
   Future<ConfiguracionAlarmas> posponerEjecucion(
     String alarmaId,
     DateTime ejecucion,