feat: add profile picture handling and auto-create UserProfile

[emidiovaleretto]

Context

UserProfile had no profile picture, and profiles weren't being created
automatically when a user signed up (they had to be created manually).
This PR addresses both.

What

• Adds a picture_profile ImageField to UserProfile
• Resizes uploaded images to a standard width (400px) and converts them to JPEG before saving
• Auto-creates a UserProfile whenever a new User is created, via a post_save signal
• Ignores the media/ directory in git (user uploads shouldn't be versioned)

Why

Picture handling: users upload images of any size/format. Normalizing
to a fixed width and a single format (JPEG) keeps storage consistent and
avoids broken layouts. Resizing happens in the model's save(), so every
save path (API, admin, shell) goes through it.

Auto-create profile: a UserProfile should always exist for a User.
Using a post_save signal guarantees this regardless of how the user is
created. Discovered while testing — creating a user left it without a profile.

How it works

• accounts/utils.py: resize_image (proportional resize, LANCZOS) and
  convert_type_image (Pillow → JPEG → Django file via BytesIO/ContentFile)
• UserProfile.save(): resizes + converts before persisting, only when an image is present
• accounts/signals.py: create_user_profile receiver, registered in apps.py ready()

Out of scope

• Placeholder image for users without a photo (next)
• PNG-with-transparency edge case in JPEG conversion (next)
• External storage (Cloudinary) for production — separate sprint

How to verify

• Upload a large image via admin → it's saved resized (~400px) as .jpg under media/photos/...
• Create a new user → a UserProfile is created automatically

Relates to #35