test: add coverage for picture_profile feature

[emidiovaleretto]

Context

The picture_profile feature (upload, resize, JPEG conversion, placeholder,
auto-creation via signal) had no test coverage. This adds it and along the
way the tests exposed a real bug with transparent PNGs.

What

Added 5 tests in tests/test_user_profile.py:

• resize_image shrinks large images to 400px width
• convert_type_image produces a real JPEG (checks actual format, not just filename)
• the serializer returns the placeholder URL when a profile has no picture
• a UserProfile is auto-created via signal when a User is created
• transparent PNGs (RGBA) are converted to a valid JPEG

Also added tests/helpers.py with a reusable create_test_image helper.

Bug fix included

The RGBA test revealed that convert_type_image crashed on transparent images
(cannot write mode RGBA as JPEG), since JPEG has no alpha channel. Fixed by
converting any non-RGB image to RGB before saving as JPEG. This would have
broken in production for any user uploading a PNG with transparency.

How to verify

pytest tests/test_user_profile.py -v — all 5 pass. Full suite stays green.

Closes #39