feat: add Gradio web UI

- app.py: Gradio Interface wrapping BoschProductRAG.query()
- Index built once at startup (not per-request)
- Markdown output: answer + linked sources + latency
- Example prompts included for quick demo
- Verified: launches on :7860, returns correct answers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitignore

@@ -10,3 +10,6 @@ build/
 metrics_report.json
 .faiss_index/
 .vector_store/
+
+.gradio/
+flagged/