Enforce a minimum 75% unit-test coverage gate

[kgdunn]

Goal

Raise unit-test coverage and enforce a minimum of 75% in CI. (Deferred for now — getting a working UI came first.)

Scope

• Add pytest-cov; run pytest --cov=scorepilot --cov-report=term-missing --cov-fail-under=75.
• Backfill backend unit tests to reach the threshold, focusing on:
  • core/ edge cases (datetime/qualitative inference, neglog/logit/power domains, pareto scaling, apply_spec with Y blocks and missing data).
  • api/ error paths and Excel multi-sheet selection.
• Wire the coverage gate into the CI workflow (CI: GitHub Actions pipeline (lint, types, tests, frontend build) #3).
• Consider a separate frontend coverage target once component tests exist.

Roadmap item — keep open.