Introduce optional web search enrichment flow for chat and regenerate requests. New /websearch endpoint calls enrich_prompt via SearXNG and returns enriched_prompt + citation sources.
DB: add sources_json column to chat_messages via ensure_sources_column migration helper.
Backend: persist sources_json for assistant replies (streaming and non-streaming); extend ChatRequest/RegenerateRequest to accept enriched_message and sources; history endpoint returns sources.
Frontend: add toggle for web search, settings for SearXNG URL + engines, and optional enrichment calls in sendMessage/regenerate. Render citation sources as rounded chips labeled with base domain under assistant replies.
Dependencies: add beautifulsoup4, httpx[http2], numpy for enrichment pipeline.
