docs: document patent PDF storage, add FileNotFoundError, commit lockfile

- Add docstring to analyze_single_patent explaining the PDF prerequisite
- Raise FileNotFoundError with helpful message when PDF is missing
- Add patent PDF storage section to README with Docker volume mount example
- Commit frontend/package-lock.json for reproducible builds

Closes leeworks-agents/SPARC#15
Closes leeworks-agents/SPARC#17

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -54,6 +54,21 @@ docker-compose up -d
 # - API Docs: http://localhost:8000/docs
 ```
 
+#### Patent PDF Storage
+
+The API stores downloaded patent PDFs in a `patents/` directory. In Docker,
+this is mounted as a bind mount (`./patents:/app/patents`) so that PDFs persist
+across container restarts.
+
+If you deploy to a different environment, ensure the `patents/` directory is a
+persistent volume. Without it, PDFs will be re-downloaded on every analysis.
+
+```yaml
+# docker-compose.yml excerpt
+volumes:
+  - ./patents:/app/patents
+```
+
 ### NixOS
 
 ```bash