Fix F541 lint failure in the event study summary

A conditional clause in the summary sentence carried an f prefix with no
placeholders, failing `ruff check app/` and blocking the deploy. Literal only;
the rendered text is unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/services/event_study_service.py

@@ -282,7 +282,7 @@ async def run_event_study(
         f"{metrics['events']} of {reliability['events_detected']} detected corrections "
         f"fall in the test period"
         + (
-            f"; too few to read recall as a property of the score."
+            "; too few to read recall as a property of the score."
             if reliability["underpowered"]
             else "."
         )