Refactor DiffPool out of server.py (Fixes #243)

[Tanishk008]

This PR resolves Issue #243.

Modifications:

• Created server/diff_pool.py to cleanly encapsulate the worker process pool management, MAX_DIFFS_PER_WORKER resets, and concurrent execution retries.
• Removed the heavily coupled diff execution and shutdown logic from server.py and DiffHandler.
• The DiffHandler HTTP routes now simply delegate execution to the DiffPool object initialized inside make_app().

[Mr0grog]

Hi @Tanishk008, I am at a conference this week and won’t have time for a closer look for a few days, but can you please make sure the tests and CI tasks pass.