feat(especialidades): especialidad model, admin, API CRUD endpoints

[DylanOsdev]

Description

Implement the especialidades app with:

• Especialidad model (nombre, descripcion, activo) extending BaseModel
• EspecialidadAdmin with list_display, search, and filter
• EspecialidadSerializer for API serialization
• CRUD function-based views with pagination
• URL routing for /api/especialidades/
• Full test suite (models, admin, serializers, API integration tests)

Acceptance Criteria

• Especialidad model with soft-delete (activo field)
• Admin registration with search and filter
• CRUD API endpoints with pagination
• All especialidades tests pass