Make copy operation atomic with explicit transaction handling

- Wrapped copy operation in try/except with explicit rollback
- Added comments explaining atomicity
- Ensures all-or-nothing behavior for copying to multiple dates

backend/routes/availability.py

@@ -160,8 +160,10 @@ async def copy_availability(
             detail=f"No availability found for source date {request.source_date}",
         )
     
-    # Copy to each target date
+    # Copy to each target date within a single atomic transaction
+    # All deletes and inserts happen before commit, ensuring atomicity
     copied_days: list[AvailabilityDay] = []
+    try:
         for target_date in request.target_dates:
             if target_date == request.source_date:
                 continue  # Skip copying to self
@@ -187,7 +189,12 @@ async def copy_availability(
             
             copied_days.append(AvailabilityDay(date=target_date, slots=target_slots))
         
+        # Commit all changes atomically
         await db.commit()
+    except Exception:
+        # Rollback on any error to maintain atomicity
+        await db.rollback()
+        raise
     
     return AvailabilityResponse(days=copied_days)