CI: coverage thresholds and reporting

[0xMegie]

Area: turbo.json, package vitest configs, CI workflow

Goal: Enforce a baseline coverage gate without blocking interns.

Tasks:

• Add coverage config for packages/contracts and apps/web.
• Set conservative thresholds agreed with maintainers.
• Document how to view coverage locally.

Acceptance:

• bun run --cwd apps/web test -- --coverage passes.
• Coverage thresholds are documented and not set above current levels.