Graphqh: Automatically set running=false when timer has counted down to 0

[gustavotrott]

As highlighted by @Scroody, the issue at hand is that Timer.running remains set to true indefinitely, even after the timer has counted down to 0. The proposed change in this PR is to adjust the timer's behavior so that running becomes false once the timer reaches its conclusion.

Additionally, the PR includes a refactoring of column names: changing startedAt to startedOn and endedAt to endedOn. This alteration aims to standardize naming conventions within the database, where On is used for bigint values and At indicates PostgreSQL timestamps.

[sonarcloud]

Quality Gate failed

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C Reliability Rating on New Code (required ≥ A)

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[github-actions]

Automated tests Summary

✅ All the CI tests have passed!

[Scroody]

Just finished testing and looks like it's working fine. Great job.