fix: Add author field to ContentSerializer for Forums V2 API compatibility

[blarghmatey]

Problem

The Forums V2 API responses were missing the author attribute that is expected by the frontend, causing issues when displaying comment/thread author information.

Diagnosis

After investigating API responses and comparing with the expected format:

• The ContentSerializer already has a username field mapped to author_username
• Frontend code expects an author field in API responses
• The field was missing from the DRF serializer output
• Both author and username should reference the same source data (author_username)

Solution

Added the author field to the ContentSerializer class, mapped to the same author_username source as the existing username field.

Changes

• Added author = serializers.CharField(source="author_username", allow_null=True) to ContentSerializer in forum/serializers/contents.py

Testing

This change:

• ✅ Maintains backward compatibility (existing username field unchanged)
• ✅ Adds the expected author field to API responses
• ✅ Uses the same source data (author_username) for consistency
• ✅ Minimal change - single line addition

Related Issues

This fixes compatibility issues with Forums V2 API responses where the frontend expects an author field in addition to the username field.

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[taimoor-ahmed-1]

@blarghmatey can you kindly fix the tests?