feat: add pdf_url field to paper metadata and update related functionality

docs: add design spec for pdf_url paper metadata field

feat: optimize database operations with batch upserts and mark inactive functionality

fix: correct real acl-anthology API usage in anthology_source

fix: don't stub missing snapshot files with empty placeholders

fix: catch huggingface_hub's real EntryNotFoundError on missing files

refactor: use openai package for the embedding API client

fix: correct FAISS id mapping across syncs, atomic snapshot upload, startup logging, health timestamp, real anthology field access

feat: wire up weekly delta-sync entrypoint

feat: add HF Hub dataset repo download/upload for index snapshot

feat: add delta computation for incremental sync

feat: read paper records from acl-anthology package

feat: add embedding API client for Qwen3-Embedding-4B

chore: scaffold project structure