feat: Add optional backend ingest router

[coderabbitai]

Summary

Add an optional minimal ingestion-callback proxy router to reduce Langflow's reachable network surface.

Details

• New FastAPI/uvicorn router at src/app/router_app.py exposing POST /internal/ingest/chunks and GET /health
• Controlled via OPENRAG_BACKEND_ROUTER_ENABLE env var (default: false)
• Started as a daemon thread on a configurable port (default: 8100)
• Langflow ingest callback URL switches to the router when enabled
• Deployment updates: Dockerfile, docker-compose, Helm charts, and operator manifests
• Unit tests added in tests/unit/test_backend_router.py

References

• PR: feat: Add optional backend ingest router #1720
• Requested by @edwinjosechittilappilly