fix: address final-review hygiene items (pydantic config, cookie alias, argon2 exception scope)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/schemas.py

@@ -1,6 +1,6 @@
 import uuid
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class RegisterRequest(BaseModel):
@@ -13,8 +13,7 @@ class UserOut(BaseModel):
     id: uuid.UUID
     username: str
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
 
 
 class LoginRequest(BaseModel):