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/services/auth.py

@@ -12,6 +12,7 @@ from rehearsalhub.config import get_settings
 from rehearsalhub.db.models import Member
 from rehearsalhub.repositories.member import MemberRepository
 from rehearsalhub.schemas.auth import RegisterRequest, TokenResponse
+from rehearsalhub.services.avatar import AvatarService
 
 
 def hash_password(plain: str) -> str:
@@ -47,11 +48,22 @@ class AuthService:
     async def register(self, req: RegisterRequest) -> Member:
         if await self._repo.email_exists(req.email):
             raise ValueError(f"Email already registered: {req.email}")
+        
+        # Create member without avatar first
         member = await self._repo.create(
             email=req.email.lower(),
             display_name=req.display_name,
             password_hash=hash_password(req.password),
         )
+        
+        # Generate default avatar for new member
+        avatar_service = AvatarService()
+        avatar_url = await avatar_service.generate_default_avatar(member)
+        
+        # Update member with avatar URL
+        member.avatar_url = avatar_url
+        await self._session.flush()
+        
         return member
 
     async def login(self, email: str, password: str) -> TokenResponse | None: