chore: add DATABASE_URL to .env.example with usage note

chore: clean up .env.example — remove duplicates, add sections

Merge remote-tracking branch 'origin/ntp' into phuc-dev

feat(auth+db): session-aware nav, ownership fix, admin hardening, schema cleanup

feat: structured PDF parsing with ParsedPaper schema (Priority 2)

merge: integrate ldh into main

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Merge remote-tracking branch 'origin/main' into phuc-dev

updated env template

feat(backend): Supabase-backed storage, observability, and DB bug fixes

docs: add Docling OCR evaluation with Kaggle server approach

feat: add per-job file logging, Docling fallback provider, paper title extraction

Improve email templates and review links

Initial implementation of AI research paper review system