Complete remaining tasks for graded assessments feature before merging into main

[seidior]

Describe the Feature / Enhancement

We need to complete the following tasks before merging the graded assessments feature branch into main (in no particular order):

• Consolidate the dummy data in api/src/assets/data.ts, then merge it into the test files for assessmentsRouter.ts and assessmentsService.ts, deleting the dummy data file.
• Remove the assessmentsDummy-related files, since that was developed only for testing purposes and is no longer needed.
• Address any remaining // TODO: comments in the code.
• Remove any boilerplate code that was added for features that have been pushed out of scope, such as for the "completion certificate" sub-feature.
• Conduct any manual tests, ensuring that all features work as they should, supplementary to our automated unit tests.
• Verify any function header comments are up to date with the functions they describe in assessmentsService.ts.
• Make any final adjustments to the SQL migration files necessary before the live site gets permanently migrated on deploy.

The final step:

• Upgrade any dependent packages and ensure the upgrades do not break our feature.

Additional Details and Resources

Notes for myself for later:

• Luxon's time duration instead of our own
• Dummy data consolidation
• Versioning
• HTTP statuses

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