Make ahnlich-ai extensible (2): Use different vector databases

Look into using other vector databases as its backend (not just ahnlich-db) e.g [Qdrant](https://github.com/qdrant/qdrant)?

The issue names ahnlich-ai, ahnlich-db, and Qdrant but no files or tests. Start by mapping how ahnlich-ai currently depends on ahnlich-db and compare that boundary with Qdrant's interface. Done means another vector database can serve as the backend without losing the existing ahnlich-db support.

Written by the indexing model from the issue text.