Mint should check pending proofs and expired quotes regularly

[KvngMikey]

fixes #596

Summary

• This PR implements proper expiration handling for mint and melt quotes that remain in the PENDING state beyond their valid lifetime. It introduces a background sweep that marks these stale quotes as EXPIRED, ensuring they can no longer be redeemed or incorrectly matched by later processing.

Changes

• Pending mint/melt quotes were never expired, which could accumulate indefinitely and cause incorrect accounting or reuse, so now we expire them
• The system had no centralized regular task to sweep and update expired quotes, this was introduced to handle this.
• All test passs.

Question

• Should we add an expiry column to mint_quotes and melt_quotes? Right now, expiry is determined implicitly using:
  created_time + EXPIRY_SECONDS (5minutes, a fixed lifespan and global timeout)..
  Please advise which approach you prefer so I can update accordingly.

[KvngMikey]

based on a conversation had with @callebtc, I’ve updated the PR so that _run_regular_tasks() strictly runs as a simple infinite loop, performing the periodic check and then sleeping. All other logic has been removed from this function as requested to be implemented after a future conversation, thank you

[callebtc]

thank you!

• please remove the mint_regular_tasks_enabled toggle from settings.py, we should run the tasks by default
• add a simple while True
• remove changes to unrelated files