fix: resolve PDF upload failures

[overhyped-pratham]

Summary

• Use a stable repo-root uploads/ path via __dirname so Express and the RAG service read the same file regardless of process working directory.
• Restore Zod askSchema / summarizeSchema exports and validation responses expected by the API tests.
• Relax multer PDF detection to rely on the .pdf extension (fixes browsers that send application/octet-stream).
• Repair invalid package.json (missing comma) and add the zod dependency.

Test plan

• npm test (14/14 passing)
• Start RAG service (uvicorn on port 5000) and Express (node server.js on port 4000)
• Upload a PDF from the frontend and confirm session_id is returned
• Ask a question against the uploaded document

[FireFistisDead]

@overhyped-pratham can't accept without issue generatioin