fix(core): reset task TTL on activity

[Amit-Amitt]

Closes #329

Summary

Task TTL cleanup was measured from createdAt, causing long-running tasks to be removed even when they were actively reporting progress.

This changes TTL cleanup to measure inactivity from lastUpdatedAt instead.

Since updateProgress() updates task status and refreshes lastUpdatedAt, progress now effectively resets the TTL.

Changes

• Use lastUpdatedAt instead of createdAt when checking task expiration.
• Add a regression test verifying that progress keeps an active task alive past its original creation-based TTL.
• Add a test verifying that inactive tasks still expire normally.

Testing

• npm run build passes.
• Full core test suite passes: 50 test suites, 579 tests.
• Added TTL cleanup regression coverage.

Notes

npm run lint could not be run because the current @nitrostack/core package does not define a lint script.