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

fix(stt): resolve post-transcription silence and harden gateway detection

feat(hardening): implement anti-silence, gateway loop protection and deep trace logging

Fix Post-STT Blockage: Async chain, Intent detection, and Feedback fallback

feat(audio): Complete robust outbound/inbound pipeline and async AI generation

fix: Split Webhook into Gateway (HuggingFace) and Processor (Railway), fix missing https:// in API_URL

feat: implement STT inbound audio, TTS fallback, and Interactive Lists

feat(arch): interactive LIST menu, audio download-media job, OpenAI 429 fallback, DAY action IDs