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/schemas/member.py

@@ -33,3 +33,4 @@ class MemberSettingsUpdate(BaseModel):
     nc_url: str | None = None
     nc_username: str | None = None
     nc_password: str | None = None  # send null to clear, omit to leave unchanged
+    avatar_url: str | None = None  # URL to user's avatar image