Add admin panel backend: upload/edit/delete endpoints + .docx support

Fix Resend 403: add User-Agent header (Cloudflare blocks Python-urllib default)

Fix email: Resend HTTP API (primary) + SMTP fallback — works on HF free tier

Upgrade Next.js 15.1.3 → 15.5.19 (CVE-2025-66478 patch)

Add Hugging Face Space config front matter to README

Initial commit: Negoptim AI RAG chatbot (backend + frontend + deploy config)