Fix/reprocessing existing results

[mihow]

Partial fixes for #718

Existing tests are passing now, but in practice images are still being reprocessed and adding duplicate classifications to existing detections.

TODO:

• add a new test that can reproduce this bug
• don't add duplicate classifications even if images are reprocessed
• update ML backend to handle skips on its own

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: ami/ml/models/pipeline.py

Function	Unhandled Issue
collect_images	ValueError: Must specify a collection, deployment or a list of images a... Event Count: 1

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