Add explorer S3 cleanup GitHub Action and clean-explorer-bucket CLI flags (#280)

[ywkim312]

Summary

Implements issue #280: run explorer map CSV cleanup from GitHub (manual trigger) using the same rules as flask clean-explorer-bucket, and make the command safe for CI.

Changes

• Workflow .github/workflows/clean_explorer_s3.yml: workflow_dispatch only; optional dry run input; runs flask clean-explorer-bucket --non-interactive (or with --dry-run). Uses the same AWS secrets as other workflows (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_BUCKET_NAME, AWS_REGION).
• clear_s3_bucket (backend/oeps/clients/s3.py): dry_run mode (print keys, no delete); safer default for objs_to_keep.
• clean-explorer-bucket (backend/oeps/commands/clean_explorer_bucket.py): --non-interactive (fail if explorer/config/sources.json is missing—no input() hang in Actions); --dry-run; docstring path fixed to explorer/config/sources.json.
• CHANGELOG.md: documents Implement S3 bucket cleanup as github action #280 (and other existing [Unreleased] bullets per current file).

How to test

1. Local (needs AWS env vars for your bucket)
   From backend/ with FLASK_APP=oeps:

   pip install -e .
   flask clean-explorer-bucket --non-interactive --dry-run

Confirm it lists Would delete … lines and exits 0.
Without credentials, expect boto/S3 errors—that only proves AWS isn’t configured.

2. Missing sources.json + non-interactive
   Point --explorer-path at a folder with no config/sources.json:

flask clean-explorer-bucket --non-interactive --explorer-path /path/to/empty-explorer-root
Should exit with an error and not prompt.

3. After merge
   Actions → Clean explorer S3 bucket → Run workflow; try dry run first, then a real run if the log looks correct.

Closes

Closes #280.

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