CI: run tests in parallel with lint (remove needs: lint)

[NWelde]

Summary

The tests job declares needs: lint, which makes the entire test matrix
wait for RuboCop and — more importantly — be skipped entirely whenever
RuboCop reports any offense. Since each test job does its own checkout, gem
install, and Postgres service, it doesn't consume anything lint produces. This
PR removes the dependency so lint and tests report independently.

Why this matters (verified on real infra)

I forked this repo and ran ci.yml both ways. With a single redundant
# rubocop:disable directive present (the kind of nit that lands in PRs):

• With needs: lint → lint fails, 0 / 186 test jobs run (run)
• Without it → lint still fails, but 185 / 186 test jobs run and pass (run)

So today, any RuboCop offense silently hides the entire test suite — a
contributor learns their formatting is off but nothing about whether their code
works.

On a clean, fully-green pipeline (lint nit fixed on both sides, representative
subset, only this line differing), removing the edge is also faster — tests start
immediately instead of waiting for lint:

	Result	Tests start	Wall-clock
needs: lint	✅ 7/7	18s	89s (run)
without	✅ 7/7	3s	77s (run)

Safety

lint still runs on every push/PR and still reports pass/fail — it just no longer
blocks or hides the tests. The change is one line.