fix(audit): timestamp Optional para evitar 500 con registros NULL

Algunos registros de audit_log tienen timestamp=NULL en DB.
AuditLogOut tenia timestamp: datetime (no opcional) causando
ValidationError -> 500 Internal Server Error al listar el audit log.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/audit.py

@@ -2,7 +2,7 @@
 
 import uuid
 from datetime import datetime
-from typing import Any
+from typing import Any, Optional
 
 from pydantic import BaseModel, ConfigDict
 
@@ -16,7 +16,7 @@ class AuditLogOut(BaseModel):
     action: str
     entity_type: str | None = None
     entity_id: str | None = None
-    timestamp: datetime
+    timestamp: Optional[datetime] = None
     details: dict[str, Any] | None = None
 
     model_config = ConfigDict(from_attributes=True)