## Problem/Motivation An AI provider can already opt in to become the **default provider** for a given operation type when none is set yet (the provider declares this and the AI module picks it up automatically if no default exists). The AI module also supports setting a **default vector database (VDB) provider**, but there is currently **no equivalent opt-in mechanism** for VDB providers. A VDB provider cannot declare "make me the default vector database if none is set", so the default always has to be configured manually. ## Proposed resolution Add the same opt-in capability for vector database providers that AI providers already have: allow a VDB provider to indicate it is willing to be the default, and have the AI module set it as the default vector database automatically **only when no default is currently configured**. Mirror the existing AI-provider implementation as closely as possible for consistency. ## Remaining tasks - [ ] Add an opt-in flag/method for VDB providers to volunteer as the default. - [ ] Set the volunteering provider as the default VDB when none is configured. - [ ] Never override an explicitly configured default.