Fix activity dedup crash: MultipleResultsFound on same-day same-sport activities

The previous query used >= on start_time with no upper bound, so it matched
ALL activities of that sport type starting after the given minute on that day —
crashing with MultipleResultsFound whenever two such activities existed.

Fix: bound the window to ±60s from start_time and use .scalars().first()
so the query returns at most one Activity rather than raising on duplicates.

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

backend/app/workers/tasks.py

@@ -82,15 +82,16 @@ def process_activity_file(self, file_path: str, user_id: int, source_type: str,
     with SyncSessionLocal() as db:
         start_time = datetime.fromisoformat(parsed["start_time"])
 
-        # Deduplicate by (user_id, sport_type, start_time date + within 60s)
+        # Deduplicate: same user + sport_type + start_time within ±60s
+        from datetime import timedelta
         existing = db.execute(
             select(Activity).where(
                 Activity.user_id == user_id,
                 Activity.sport_type == parsed["sport_type"],
-                func.date(Activity.start_time) == start_time.date(),
+                Activity.start_time >= start_time - timedelta(seconds=60),
-                Activity.start_time >= start_time.replace(second=0, microsecond=0),
+                Activity.start_time <= start_time + timedelta(seconds=60),
             )
-        ).scalar_one_or_none()
+        ).scalars().first()
 
         if existing:
             # Stamp garmin_activity_id if this came from a Garmin Connect sync