Move slot expansion logic to ExchangeService

- Add get_available_slots() and _expand_availability_to_slots() to ExchangeService
- Update routes/exchange.py to use ExchangeService.get_available_slots()
- Remove all business logic from get_available_slots endpoint
- Add AvailabilityRepository to ExchangeService dependencies
- Add Availability and BookableSlot imports to ExchangeService
- Fix import path for validate_date_in_range (use date_validation module)
- Remove unused user_repo variable and import from routes/invites.py
- Fix mypy error in ValidationError by adding proper type annotation

backend/repositories/user.py

@@ -21,3 +21,12 @@ class UserRepository:
         """Get a user by ID."""
         result = await self.db.execute(select(User).where(User.id == user_id))
         return result.scalar_one_or_none()
+
+    async def get_godfather_email(self, godfather_id: int | None) -> str | None:
+        """Get the email of a godfather user by ID."""
+        if not godfather_id:
+            return None
+        result = await self.db.execute(
+            select(User.email).where(User.id == godfather_id)
+        )
+        return result.scalar_one_or_none()