fix: add last_login to AdminUserResponse schema and builder

The last_login field was missing from the API response, causing it
to always show empty on the admin user detail page.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/admin_users.py

@@ -59,6 +59,7 @@ class AdminUserResponse(BaseModel):
     platform_assignments: list[PlatformAssignmentResponse] = []
     created_at: datetime
     updated_at: datetime
+    last_login: Optional[datetime] = None
 
     class Config:
         from_attributes = True
@@ -126,6 +127,7 @@ def _build_admin_response(admin: User) -> AdminUserResponse:
         platform_assignments=assignments,
         created_at=admin.created_at,
         updated_at=admin.updated_at,
+        last_login=admin.last_login,
     )