Remove unused backend install from Build workflow

[gkorland]

Parent issue

Part of #550 (CI Pipeline Optimization Analysis)

Problem

The Build workflow (nextjs.yml) installs backend Python dependencies with pip install -e ".[test]" (35s) but never runs any backend tests or linting. This step accounts for 55% of the Build workflow runtime.

Solution

Either:

• Remove the Setup Python and Install backend dependencies steps entirely (if backend validation is not needed in this workflow)
• Or add backend unit tests / pylint to justify the install

Estimated savings

~35s per Build run

Effort

Trivial