Backend: Qdrant integration (formalize from PoC)

[jongalloway]

This issue tracks formalizing the Qdrant backend integration based on the existing Aspire demo PoC.

Goals

• Promote the Aspire demo PoC into a formal backend implementation (package or folder) with clear registration and configuration.
• Provide configuration templates and docs for local and hosted Qdrant.
• Add integration tests (guarded by opt-in environment variables/docker) and a minimal demo scenario.

Tasks

• Create NLWebNet.Backend.Qdrant (project/folder) implementing IDataBackend (multi-backend ready) and vector operations.
• Configuration schema and binding for Qdrant (endpoint, API key, collection/schema settings).
• Service registration extension method and sample config_retrieval.yaml entries.
• Integration tests with docker-compose or Testcontainers (optional) to validate basic CRUD/vector search.
• Documentation: setup (local docker + cloud), schema expectations, troubleshooting.
• Wire into demo sample with feature flag.

Acceptance Criteria

• Qdrant backend can be enabled via config and used alongside other backends.
• Vector insert/search APIs work with the NLWebNet abstractions; dedupe compatible.
• Sample + docs allow a user to run end-to-end locally with Docker.

Link

• Parent: 🗄️ Advanced Backend Support #40 Advanced Backend Support