Delegate availability persistence to AvailabilityRepository

- Add create() and create_multiple() methods to AvailabilityRepository
- Update AvailabilityService to use repository methods instead of direct db operations

backend/repositories/availability.py

@@ -39,3 +39,28 @@ class AvailabilityRepository:
         await self.db.execute(
             delete(Availability).where(Availability.date == target_date)
         )
+
+    async def create(self, availability: Availability) -> Availability:
+        """
+        Create a new availability record.
+
+        Args:
+            availability: Availability instance to persist
+
+        Returns:
+            Created Availability record (flushed to get ID)
+        """
+        self.db.add(availability)
+        await self.db.flush()
+        return availability
+
+    async def create_multiple(self, availabilities: list[Availability]) -> None:
+        """
+        Create multiple availability records in a single transaction.
+
+        Args:
+            availabilities: List of Availability instances to persist
+        """
+        for availability in availabilities:
+            self.db.add(availability)
+        await self.db.flush()