ensure_index() opened a Chroma client to check for documents, which created
an empty database and held it open. chromadb caches clients per path, so the
subsequent rebuild deleted the directory out from under the cached client and
Chroma.from_documents crashed with sqlite 'unable to open database file'
(code 14) on fresh deployments. Check for chroma.sqlite3 on disk instead.
Also remove the unused Dockerfile left over from the Space template (the
Space runs with sdk: streamlit from README frontmatter; the Dockerfile
referenced a nonexistent src/streamlit_app.py).
