build(deps): add watchfiles to pyproject + refresh uv.lock

Promotes watchfiles from a transitive dep (uvicorn[standard]) to a
direct dep so that uv-based installs pull it in even when uvicorn's
optional extras are unselected. `requirements.txt` already listed it.

The lockfile churn beyond watchfiles is pre-existing drift: mem0ai was
added to pyproject.toml in #598 without re-locking, so `uv lock --check`
was already failing on main before this commit.