Make sure `uv` can generate a lockfile

*See [parent issue](https://github.com/element-hq/synapse/issues/19566) for more context*

Two PRs to unblock this:

- #19490 ✅ merged
- #19491 ✅ merged

`uv lock` now works on the current codebase. This issue can be closed.

Read the parent issue and the two linked PRs, #19490 and #19491, to understand the completed lockfile work. Run `uv lock` on the current codebase to confirm it succeeds; the stated done condition is that lockfile generation works, and the issue can then be closed.

Written by the indexing model from the issue text.