Migrate merge_authors_json to FastAPI

[Sanket17052006]

Closes #

This PR migrates the merge_authors_json web.py handler to FastAPI.

Technical

• AddMergeAuthorsBody Pydantic model for request validation
• Uses shared LibrarianDep dependency injection for auth
• Replaced web.ctx.site → site.get() ContextVar in merge_authors.py and edits.py
• Marked legacy class with @deprecated

Testing

docker compose run --rm home pytest openlibrary/tests/fastapi/test_merge_authors.py -v

Login:

curl -s -c /tmp/cookies.txt -X POST "http://localhost:8080/account/login.json" \
  -H "Content-Type: application/json" \
  -d '{"username":"openlibrary","password":"openlibrary"}'

POST merge authenticated → 200:

curl -s -b /tmp/cookies.txt -X POST "http://localhost:18080/authors/merge.json" \
  -H "Content-Type: application/json" \
  -d '{"master": "/authors/OL1A", "duplicates": ["/authors/OL2A"]}'

POST unauthenticated → 401:

curl -s -w "\nHTTP %{http_code}" -X POST "http://localhost:18080/authors/merge.json" \
  -H "Content-Type: application/json" \
  -d '{"master": "/authors/OL1A", "duplicates": ["/authors/OL2A"]}'

Screenshot

Stakeholders

@RayBB

[RayBB]

Carefully tested this locally then on testing. It's kind of annoying to test there because you need to craft a fetch to run appropriately. But it did work great!

Thanks for getting this one done and it seems to be much much faster!