Migrate api/link endpoint to FastAPI

[Sanket17052006]

Closes #

This PR migrates link_ia_ol (/api/link) to FastAPI

Technical

• Add POST /api/link FastAPI endpoint at openlibrary/fastapi/link.py
• HMAC verification via HMACToken.verify (unix_time=True) matching legacy behaviour
• Edition association via accounts.RunAs("ImportBot") + site.save()
• Legacy link_ia_ol class in openlibrary/plugins/openlibrary/api.py marked @deprecated("migrated to fastapi")

Testing

• Ran make test-py-uv PYTEST_ARGS="openlibrary/tests/fastapi/test_link.py -v"
• Add - IA_SYNC_SECRET=${IA_SYNC_SECRET:-} to compose.override.yaml under fast_web.environment:
• Restart and verify:
  IA_SYNC_SECRET=test-secret docker compose up -d --force-recreate fast_web

python3 << 'PYEOF'
import hmac, hashlib, time
secret = 'test-secret'
msg = f'ocaid-test|OL1M|{int(time.time())+3600}'
d = hmac.new(secret.encode(), msg.encode(), hashlib.sha256).hexdigest()
print(f"curl -X POST http://localhost:18080/api/link -d \"digest={d}&msg={msg}\" -w \"\\nHTTP %{{http_code}}\"")
PYEOF

Verify Scenarios - Valid HM, Bad/forged digest, Expired timestamp and No IA_SYNC_SECRET set

Screenshot

N/A

Stakeholders

@RayBB