[Code Embeddings] Add last_queried_at column in ActiveContext::Code::Repository record

Context

From the description in &19655:

We could store a timestamp like queried_at on the active context project record and stop indexing the project after certain time and potentially delete it from the collection completely if no one is asking questions about it for a prolonged period of time.

Proposal

Add last_queried_at timestamp to the p_ai_active_context_code_repositories table. (model: Ai::ActiveContext::Code::Repository).

This will be used in both the Embeddings Indexing and Duo Chat Integration changes needed to support the auto-deletion workflow, so this should be raised as its own MR.