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>

pyproject.toml

@@ -7,6 +7,7 @@ requires-python = ">=3.11"
 dependencies = [
     "beautifulsoup4>=4.15.0",
     "faiss-cpu>=1.15.0",
+    "gradio>=6.27.0",
     "langchain>=1.4.0",
     "langchain-community>=0.4.2",
     "langchain-google-genai>=4.4.0",