Add Return Type Hints to Model Manager Methods

Add Return Type Hints to Model Manager Methods
Estimated effort: 2 points
Files: backend/apps/owasp/models/managers/*.py, backend/apps/github/models/managers/*.py

Model managers like ActiveChapterManager, OpenIssueManager, etc., have methods missing return type hints:

get_queryset() should return models.QuerySet
Property methods like without_geo_data, assignable need type hints
Example:

def get_queryset(self) -> models.QuerySet:
"""Get queryset."""

Parent Issue: #2646