Switch to uv for dependency management

[ojies]

Problem

The project currently uses pip + requirements.txt with no lockfile, meaning installs are not reproducible across machines, and dependency resolution failures (like the training issues in #73) are hard to diagnose.

Proposed Solution

Replace pip with [uv](https://github.com/astral-sh/uv), a drop-in pip replacement that is significantly faster and generates a uv.lock lockfile for fully reproducible installs.

No changes to the existing requirements.txt needed to get started. Migration path:

pip install uv
uv pip install -r requirements.txt   # works as-is

Longer term, migrate to pyproject.toml + uv.lock for proper dependency pinning.