Improve scientific RAG citations and retrieval context

[sureshchouksey8]

/claim #45

Summary

• Add scientific RAG helpers for section-aware chunking, stable citation keys, bounded retrieval result counts, and retrieved-document formatting with distance context.
• Store richer PDF chunk metadata during ingestion: title, page, source, section, chunk index, per-page chunk index, and citation key.
• Use the request origin instead of hardcoded localhost for RAG document lookup, pass a configurable result count, and instruct answers to cite exact retrieved citation keys.
• Respect CHROMA_PATH in document fetches and validate empty/non-POST requests.

Verification

• npm test -- scientific-rag.test.ts --run
• ./node_modules/.bin/tsc --noEmit
• npm run lint passed with pre-existing React hook dependency warnings in unrelated files.

Notes

This focuses on grounding and citation traceability for scientific/research QA while staying within the existing Chroma/LangChain architecture.