[Enhancement] mobieusAI: multi-turn conversational memory in the Q&A drawer

What

Today each question in the mobieusAI drawer is independent — the model sees the question and the retrieved sources, nothing else. Adding multi-turn memory lets a member ask "what about admins?" after a question about moderators without restating the context, and lets the model say "earlier you asked about X" when relevant.

Where it lives

• Surface: Q&A drawer
• Backlog id: B-AI-RAG-2

Implementation notes

• New ai_assistant_conversations + ai_assistant_messages tables (per tenant)
• Drawer sends the conversation_id with every ask
• Last N turns (default N=6) included in the prompt context
• "New conversation" button resets the thread
• Per-conversation spend cap (separate from per-feature daily cap)

Tracked as B-AI-RAG-2 in docs/BACKLOG.md.