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

docs: add design spec for pdf_url paper metadata field

fix: update .gitignore to include notes.md, work/, and anthology-data/

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

feat: add FastAPI routes for keyword/similarity search and health check

feat: add search service combining keyword and similarity search

chore: scaffold project structure