[E4-7] Raise coverage thresholds — 50% line threshold too low

**Finding:** CI-3 (MEDIUM) | **Milestone:** M-E4: Reliability (#18)

**Problem:** 50% line threshold only; no branch/function enforcement.

**Fix:** Update vitest.config.ts to lines: 70, branches: 60, functions: 70, statements: 70.

**Files:** vitest.config.ts

**Acceptance:** CI fails when coverage drops below new thresholds.