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/avatar.py

@@ -0,0 +1,54 @@
+"""Avatar generation service using DiceBear API."""
+
+from typing import Optional
+import httpx
+from rehearsalhub.db.models import Member
+
+
+class AvatarService:
+    """Service for generating and managing user avatars."""
+
+    def __init__(self):
+        self.base_url = "https://api.dicebear.com/v6"
+
+    async def generate_avatar_url(self, seed: str, style: str = "identicon") -> str:
+        """Generate a DiceBear avatar URL for the given seed.
+        
+        Args:
+            seed: Unique identifier (user ID, email, etc.)
+            style: Avatar style (default: identicon)
+            
+        Returns:
+            URL to the generated avatar
+        """
+        # Clean the seed for URL usage
+        clean_seed = seed.replace("-", "").replace("_", "")
+        
+        # Construct DiceBear URL
+        return f"{self.base_url}/{style}/svg?seed={clean_seed}&backgroundType=gradientLinear&size=128"
+
+    async def generate_default_avatar(self, member: Member) -> str:
+        """Generate a default avatar for a member using their ID as seed.
+        
+        Args:
+            member: Member object
+            
+        Returns:
+            URL to the generated avatar
+        """
+        return await self.generate_avatar_url(str(member.id))
+
+    async def get_avatar_url(self, member: Member) -> Optional[str]:
+        """Get the avatar URL for a member, generating default if none exists.
+        
+        Args:
+            member: Member object
+            
+        Returns:
+            Avatar URL or None
+        """
+        if member.avatar_url:
+            return member.avatar_url
+        
+        # Generate default avatar if none exists
+        return await self.generate_default_avatar(member)