fix: declare sync_every and sync_every_tasks as class attributes, not properties (#217)

## Summary

`Scheduler.sync_every` and `Scheduler.sync_every_tasks` are declared as `@property` in `celery-stubs/beat.pyi`, but in celery's source code ([`celery/beat.py` lines 242-245](https://github.com/celery/celery/blob/main/celery/beat.py#L242-L245)) they are plain class attributes:

```python
class Scheduler:
    sync_every = 3 * 60
    sync_every_tasks = None
```

This causes `reportAssignmentType` errors in pyright when subclasses override them as class variables, which is the intended usage pattern:

```python
class MyScheduler(Scheduler):
    sync_every = 60  # error: Type "Literal[60]" is not assignable to declared type "property"
```

## Fix

Changed both from `@property` to plain class attribute declarations:

```python
# Before
@Property
def sync_every(self) -> int: ...
@Property
def sync_every_tasks(self) -> int | None: ...

# After
sync_every: int
sync_every_tasks: int | None
```

Related: #216

Author: jamesrusso

celery-stubs/beat.pyi

@@ -123,10 +123,8 @@ class Scheduler:
     def send_task(self, *args: Any, **kwargs: Any) -> Any: ...
     def set_schedule(self, schedule: dict[str, ScheduleEntry]) -> None: ...
     def should_sync(self) -> bool: ...
-    @property
-    def sync_every(self) -> int: ...
-    @property
-    def sync_every_tasks(self) -> int | None: ...
+    sync_every: int
+    sync_every_tasks: int | None
     def tick(
         self,
         event_t: type[event_t] = ...,