feat: add provision_oauth_user() to users module

Creates or updates a user from an OAuth2 provider: new users are
inserted with an empty password_hash (OAuth-only login); existing users
have their display name and avatar refreshed while all other attributes
(admin flag, password_hash, notification_channels) are preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/users.py

@@ -187,6 +187,26 @@ def authenticate(username: str, password: str) -> "User | None":
     return None
 
 
+def provision_oauth_user(username: str, full_name: str, avatar: str) -> "User":
+    """Create or update a user sourced from an OAuth2 provider.
+
+    New users are inserted with no password_hash — they can only authenticate
+    via OAuth.  Existing users (e.g. defined in config with a password) have
+    their display name and avatar refreshed; all other attributes are preserved.
+    """
+    user = users.get(username)
+    if user is None:
+        user = User(username=username, full_name=full_name, avatar=avatar)
+        users[username] = user
+        logger.info("Provisioned OAuth user %r", username)
+    else:
+        if full_name:
+            user.full_name = full_name
+        if avatar:
+            user.avatar = avatar
+    return user
+
+
 # ---------------------------------------------------------------------------
 # Session management
 # ---------------------------------------------------------------------------