Swap pdfplumber → pypdf for upload parsing (3x faster text extraction)

fix: use pdfplumber text for chunking to ensure accurate page numbers for citations

fix: keep MarkItDown for chunking, pass pdfplumber page texts separately so page numbers align for bbox extraction

production hardening: circuit breakers, graceful degradation, health checks, parallel uploads, HF Spaces deploy config

fix models.py: match Neon schema varchar sizes, add email unique constraint