Sync backend to GitHub d419447: voice transcription, builder dashboard, analytics

Report vector-store health alongside embedding health

Report embedding health, and stop skipping embeds silently

Deploy the audit remediation backend

Replace dead Groq models with a provider fallback chain

Update backend from polished-with-website branch - new routers (feedback, saved_prompts), updated services and configs

feat: update backend with complete prompt engine v4.0 - Enhanced main.py with request logging, CORS config, embedding preload - Updated config.py with tier-based model routing, rate limits, Stripe stubs - Expanded prompts.py with streaming, voice-enhance, feedback, history, usage endpoints - Added llm_service.py with Groq client pool and key rotation - Updated memory_service.py with passive learning, feedback analysis - Added migrate_embeddings.py for multilingual model migration - Updated schemas.py, auth.py, saved_prompts.py, requirements.txt - Removed prompt_engineering_skeleton directory

Update backend: mode-aware enhance, saved prompts CRUD, voice-enhance, feedback endpoint

refactor for HF spaces: moved code to backend/