chore: remove disk-upload remnants after S3 cutover

[dosaki]

⚠️ DO NOT MERGE UNTIL PR #1 IS MERGED AND VERIFIED IN PROD

This PR is the cleanup half of the disk-to-S3 cutover. The work it removes is load-bearing during the cutover and only safe to delete after the migration has actually run successfully in prod.

Merge gate checklist

• PR feat: store book photos in S3 with presigned PUT uploads #1 (feat: store book photos in S3 with presigned PUT uploads) has been merged.
• deploy.yml has finished and rolled out new ECS tasks.
• The librarian-services-api task logs (CloudWatch /ecs/librarian-services/api) include a line like Done. Uploaded N object(s); updated M row(s). from the migration script.
• At least one book cover renders in the prod UI (i.e. S3 actually serves it).

If any of those is unchecked, close this PR or wait — merging now leaves prod with the legacy shim gone and no migration to bring DB rows up to date.

What this removes

• app/static/uploads/ (24 baked-in book covers, now living in S3 under books/).
• scripts/migrate_uploads_to_s3.py — the one-shot data migration; nothing left to migrate.
• The python scripts/migrate_uploads_to_s3.py line in entrypoint.sh — would now be a missing-file error on container start.
• The legacy-filename shim from app.storage.public_url — once every row carries books/, the if "/" not in key branch is dead code that silently rewrites bad input. Removing it makes future bugs (e.g. a code path that accidentally sets photo_filename = "garbage") fail loudly instead of producing a wrong-looking S3 URL.

Test plan

• Local: flask run boots; /library/, /library/dashboard, /borrower/dashboard, /library/add, /library/<id>/edit all render covers (resolved against MinIO or staging S3).
• Post-merge: deploy.yml runs without the migrate_uploads_to_s3.py step (entrypoint goes straight from flask db upgrade to gunicorn).
• Spot-check book covers in prod UI immediately after deploy.

Base-branch note

This PR's base is feat/s3-uploads, not main. If PR #1 is merged via:

• Create a merge commit → feat/s3-uploads survives, this PR's base auto-tracks. Merge it next.
• Squash and merge with auto-delete-branch → feat/s3-uploads is deleted; this PR's base disappears. Retarget with gh pr edit 2 --base main first.

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