feat: add logic for image reference counting

[carissa-tang]

Notion ticket link

Add Reference Counting to Images

Implementation description

• Add ImageCount model, interface, implementation, and unit tests
• Update ImageUploadService
  • Initialize count on image creation
  • Decrement count on delete, and delete image only if count is 0
  • Add incrementImageCount function for test service
  • Increment count on upload of existing image (i.e. filePath exists)
• Update TestService
  • Increment image count on duplicate test

Steps to test

1. Create a test with an image. Check that the reference count is 1 and the image is uploaded to GCP.
   

2. Duplicate a test. Check that the reference count is 2. Check that the same image is still in GCP and there is no new upload.
   

3. Delete the duplicate test. Check that the reference count is 1. Check that the same image is still in GCP.
   

4. Delete the original test. Check that the record is deleted from the DB. Check that the image is deleted from GCP.

What should reviewers focus on?

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[jfdoming]

Since this runs for all images, have you done any sort of load test before/after? There's probably minimal difference considering things should theoretically run in parallel, but it would be good to verify