Add two new tables for cross-chat memory feature:
- memory_facts: stores explicit user-provided facts (user_id, content, created_at)
- conversation_summaries: stores conversation summaries with vector embeddings for similarity search
This migration prepares the database schema for Task 2-7 of the cross-chat memory feature plan.
No indexes yet (sequential scan sufficient for expected data volume).
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019iZMEPk2Kt1UC96Lo9bC5w
