fix: make account_id and account_role nullable in User type and schema

The User model has account_id as Optional[UUID] but the frontend type
and backend schema declared it non-nullable. Solo users without a team
have no account, so both must allow null.

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

backend/app/schemas/user.py

@@ -40,8 +40,8 @@ class UserLogin(BaseModel):
 class UserResponse(UserBase):
     id: UUID
     role: str = "engineer"
-    account_id: UUID
-    account_role: str
+    account_id: Optional[UUID] = None
+    account_role: Optional[str] = None
     is_super_admin: bool = False
     is_active: bool = True
     must_change_password: bool = False

frontend/src/types/user.ts

@@ -8,8 +8,8 @@ export interface User {
   is_super_admin: boolean
   is_active: boolean
   must_change_password: boolean
-  account_id: string
-  account_role: 'owner' | 'engineer' | 'viewer'
+  account_id: string | null
+  account_role: 'owner' | 'engineer' | 'viewer' | null
   created_at: string
   last_login: string | null
 }