Fix SonarQube warning: signal handler defined as instance method without self

**Context**

SonarQube reports a **Critical Code Smell** (`python:S5720`) in
`backend/apps/mentorship/signals/program.py`.

A Django `post_save` signal handler is defined inside a class but does not take
`self` and is not marked as `@staticmethod`. This makes the method appear to be
an instance method even though it is not, which is confusing and non-idiomatic
Python.

---

**Current Code**

```python
class ProgramPostSaveHandler:
"""Handles post_save signal for Program model to clear Algolia cache."""

@receiver(post_save, sender=Program)
def program_post_save_clear_algolia_cache(sender, instance, **kwargs):
logger.info(
"Signal received for program '%s'. Clearing 'programs' index.",
instance.name,
)
clear_index_cache("programs")
```

**Why this should be fixed**

- The method is not an instance method and does not use self

- Defining it this way is confusing and against Python best practices

- SonarQube flags this as a Critical maintainability issue

- The intent of the code would be clearer if the method were explicitly static

**Proposed Fix**

Mark the signal handler as a @staticmethod:

```
class ProgramPostSaveHandler:
"""Handles post_save signal for Program model to clear Algolia cache."""

@staticmethod
@receiver(post_save, sender=Program)
def program_post_save_clear_algolia_cache(sender, instance, **kwargs):
logger.info(
"Signal received for program '%s'. Clearing 'programs' index.",
instance.name,
)
clear_index_cache("programs")
```

**Scope**

- Small, isolated change

- No behavior or API changes

- Improves clarity and maintainability

- Resolves the SonarQube warning