chore: finalize Sprint P2 & P3 optimizations, baseline prisma migrations, and update technical audit docs

chore: execute Sprint 38 technical debt resolution (Type Safety, Zod validation, Vitest, Mock LLM extracted)

chore(api): add fast-levenshtein for Whisper WER calibration benchmark

chore: test API distribution

feat(api): add whisper confidence calibration script and endpoint