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 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 FAISS IndexFlatIP helpers for similarity search

feat: add SQLite metadata store with FTS5 keyword search

chore: scaffold project structure