Fix negative expires_ms and avoid worker freezing while using cron

[Matvey-Kuk]

We're encouraging issue with negative expires_ms while excessively using frequent cron jobs. It's happening rarely, but is causing worker freezes which is pretty annoying in production.

The issue is also mentioned here: #447

To reproduce:

1. Schedule a cron task.
2. Put a debugger breakpoint to https://github.com/python-arq/arq/blob/main/arq/worker.py#L752 (emulating some delay here: https://github.com/python-arq/arq/blob/main/arq/worker.py#L724)
3. It will cause _defer_until to be in the past.
4. It will cause expires_ms to be negative here: https://github.com/python-arq/arq/blob/main/arq/connections.py#L170
5. Raises

Task exception was never retrieved
future: <Task finished name='Task-5' coro=<Worker.async_run() done, defined at /Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py:315> exception=ResponseError('Command # 1 (PSETEX arq:job:cron:process_background_ai_task:1726036281123 -319 �\x04�H\x00\x00\x00\x00\x00\x00\x00}�(�\x01t�N�\x01f��\x1fcron:process_background_ai_task��\x01a�)�\x01k�}��\x02et��\x06r��ߑ\x01u.) of pipeline caused error: ("invalid expire time in \'psetex\' command",)')>
Traceback (most recent call last):
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py", line 320, in async_run
    await self.main_task
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py", line 361, in main
    await self._poll_iteration()
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py", line 401, in _poll_iteration
    await self.heart_beat()
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py", line 727, in heart_beat
    await self.run_cron(now, cron_window_size)
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/worker.py", line 759, in run_cron
    job_futures and await asyncio.gather(*job_futures)
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/arq/connections.py", line 177, in enqueue_job
    await pipe.execute()
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/redis/asyncio/client.py", line 1399, in execute
    return await conn.retry.call_with_retry(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/redis/asyncio/retry.py", line 59, in call_with_retry
    return await do()
           ^^^^^^^^^^
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/redis/asyncio/client.py", line 1287, in _execute_transaction
    self.raise_first_error(commands, response)
  File "/Users/matvey/Desktop/keep-oss/.venv/lib/python3.11/site-packages/redis/asyncio/client.py", line 1326, in raise_first_error
    raise r
redis.exceptions.ResponseError: Command # 1 (PSETEX arq:job:cron:process_background_ai_task:1726036281123 -319 ��H}�(�t�N�f��cron:process_background_ai_task��a�)�k�}��et��r��ߑu.) of pipeline caused error: ("invalid expire time in 'psetex' command",)

6. Worker will stay in the "frozen" state not executing tasks.

Thank you for the library! ❤️

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@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
- Coverage   96.27%   96.01%   -0.27%     
==========================================
  Files          11       11              
  Lines        1074     1078       +4     
  Branches      209      190      -19     
==========================================
+ Hits         1034     1035       +1     
- Misses         19       21       +2     
- Partials       21       22       +1     

Files with missing lines	Coverage Δ
arq/worker.py	97.17% <ø> (ø)

... and 1 file with indirect coverage changes

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[Matvey-Kuk]

@Tinche @JonasKs I found you authored nearby code, wonder if you could review this, thank you :)

[Matvey-Kuk]

@samuelcolvin ? :)