feat: implement user avatars with DiceBear integration

- Add avatar_url field to MemberSettingsUpdate schema
- Create AvatarService for generating default avatars using DiceBear
- Update auth service to generate avatars on user registration
- Add avatar upload UI to settings page
- Update settings endpoint to handle avatar URL updates
- Display current avatar in settings with upload/generate options

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

api/src/rehearsalhub/routers/auth.py

@@ -54,6 +54,8 @@ async def update_settings(
         updates["nc_username"] = data.nc_username or None
     if data.nc_password is not None:
         updates["nc_password"] = data.nc_password or None
+    if data.avatar_url is not None:
+        updates["avatar_url"] = data.avatar_url or None
 
     if updates:
         member = await repo.update(current_member, **updates)