Backend: Milvus integration

[jongalloway]

This issue tracks adding Milvus as a backend option for vector storage/search.

Goals

• Implement a Milvus-backed vector store for NLWebNet abstractions.
• Provide configuration and environment setup for local (docker) and hosted Milvus.
• Provide an end-to-end demo path similar to Qdrant.

Tasks

• Create NLWebNet.Backend.Milvus implementing IDataBackend and vector operations.
• Configuration schema and binding (endpoint, API key if applicable, collection/index params).
• Service registration extension and sample config entries.
• Integration tests with docker-compose or Testcontainers.
• Documentation: setup, performance notes, troubleshooting.
• Wire into demo sample with feature flag.

Acceptance Criteria

• Milvus backend can be enabled via config and used alongside other backends.
• Vector insert/search APIs work and pass integration tests.
• Sample + docs allow local run with Docker.

Link

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