Fix PRIVATE mode submissions incorrectly receiving scores

PRIVATE mode runs return timing info but should not affect leaderboard
ranking. This change restricts score computation to only PUBLIC and
SECRET modes, ensuring PRIVATE submissions have score=None as intended.

Author: Mark Saroufim

src/libkernelbot/backend.py

@@ -142,14 +142,16 @@ async def submit_leaderboard(  # noqa: C901
 
         if result.success:
             score = None
-            # Check for the mode's result key (public or secret)
             mode_key = mode.value
-            if (
-                mode_key in result.runs
-                and result.runs[mode_key].run.success
-                and result.runs[mode_key].run.passed
-            ):
-                score = compute_score(result, task, submission_id, mode_key)
+            # Only compute scores for ranked modes (PUBLIC and SECRET), not PRIVATE
+            # PRIVATE runs return timing info but don't affect leaderboard ranking
+            if mode in (SubmissionMode.PUBLIC, SubmissionMode.SECRET):
+                if (
+                    mode_key in result.runs
+                    and result.runs[mode_key].run.success
+                    and result.runs[mode_key].run.passed
+                ):
+                    score = compute_score(result, task, submission_id, mode_key)
 
             # verifyruns uses a fake submission id of -1
             if submission_id != -1: