feat: implement background task processing with Celery and Redis

style: run black formatter to fix CI

fix: enforce minimum question count and fix violation route imports

fix: comprehensive stability fixes for skip flow, follow-ups, and results page

feat: comprehensive production hardening of interview state machine and frontend-backend synchronization

Fix Interview State Machine: Sync frontend/backend contracts and enhance completion logic

harden: production audit fixes (logging, atomic writes, LLM validation, answer limits, memory leak fix)

Final updated project