Update deprecated API usage

[dentiny]

During local testing, I found certain APIs have been deprecated.

/Users/haojiang/Desktop/lance-data-viewer/backend/app.py:38: DeprecationWarning:
          on_event is deprecated, use lifespan event handlers instead.

[gordonmurray]

Thanks for the fix. The lifespan handler is the correct replacement. The test suite covers it already, because conftest.py enters the TestClient as a context manager.

I ran the full version matrix against your branch. All 30 tests pass on the seven lancedb versions, 0.3.1 to 0.33.0. The warning count drops from 3 to 1.

The last warning is the app= shortcut in httpx. This warning comes from our test pin, httpx<0.28. You do not need to fix it.

You do not need to change anything in this PR. This project wants a changelog entry for every PR. I will write that entry and open the issue for it. You have three other PRs open here already.