fix: declare sync_every and sync_every_tasks as class attributes, not properties

[jamesrusso]

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) they are plain class attributes:

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:

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:

# 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

[sbdchd]

Thank you!