Role-based controls for Space and Topic management

Space owners can now create, rename, and delete Topics via the MCP tools create_topic, update_topic, and delete_topic. Editors retain the ability to store memories and assign them to existing topics, but only owners can reshape a Space's taxonomy — preventing unintended structural changes by members without full access. The distinction is enforced at both the application layer (clear error messages) and the database layer (owner-only permission check on each RPC).