Auto-generate user avatars with no storage

Implement a deterministic default avatar system using DiceBear.

Requirements:

1. Avatar generation

* Use DiceBear HTTP API
* Base URL:
  https://api.dicebear.com/9.x/avataaars/svg
* Use a deterministic seed per user (user.id OR hashed email)
* Example:
  https://api.dicebear.com/9.x/avataaars/svg?seed={USER_IDENTIFIER}