Add task signature and locking when processing fetch_or_enqueue

[bjester]

Summary

Description of the change(s) you made

• Adds signature to task result model, which is a hash of task name, arguments, and associated channel ID, making it a consistent value for duplicate task runs
• Updates task fetching logic to utilize the new signature field
• Updates task fetch_or_enqueue logic to open a postgres advisory lock based off the task signature, to prevent concurrency issues resulting in duplicate tasks

Manual verification steps performed

1. All existing tests passed
2. Used channel edit UI to generate tasks

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References

Resolves #3836

Comments

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Contributor's Checklist

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• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[rtibbles]

One thought about key ordering that might affect stable signature generation.