Stop treating space_id IS NULL as personal

Personal Memories are now correctly identified by their personal-space UUID rather than a null space_id across all surfaces: the scope labels on the All Memories feed, Asset detail, Insight detail, and Search palette all render "Personal" correctly for both legacy null-space rows and post-migration rows stored with the personal-space id. MCP store_memory and the move_memory RPC no longer write null space_ids for personal captures — they resolve the caller's personal space first so every Memory lands in a real space row with proper membership-based access control.