- Introduce `reasoning_content`, `suggested_questions`, `citations`, and `audio_status` fields in conversation and app response schemas
- Conditionally set `audio_status` to `"pending"` only when `audio_url` is present
- Replace `model_dump` override with `@model_serializer(mode="wrap")` for cleaner serialization logic
- Change knowledge base validation failure from `RuntimeError` to warning + `continue` to avoid halting retrieval on invalid KB