Convert to a uv / pyproject.toml package

[gaurav]

Follow-up to the loader reorganization (branch overhaul-loader). Turn NodeNorm into a proper Python package managed with uv:

• Replace the three requirements*.txt files with pyproject.toml dependency groups (frontend / loader / test).
• Replace the root load.py shim with a nodenorm-load console script. Note this requires a matching change in the translator-devops Helm chart, which currently runs python load.py.
• Revisit whether the frontend and loader should be split into separate installable packages/images rather than shipping the loader in the webserver image.

Context: documentation/Development.md (backlog section).