Personal Space: topics now support rename, delete, and merge

The personal home (/app) now treats the personal vault as a real space row (kind='personal') in the database, giving it full topic management parity with Shared Spaces. A new ensure_personal_space() RPC lazily creates the personal space row on first visit and backfills any legacy null-space topics and memories into it. The personal home now shows a gear button on each non-General topic card; clicking it opens a "Manage topic" dialog with Rename, Merge into, and Delete panels. Deleted topics reassign their memories to General automatically. A new kind column on the spaces table ('team' / 'personal') gates the shared-spaces sidebar and the MCP list_spaces tool to team spaces only, keeping the personal vault private. The MCP store_memory tool resolves the personal space via ensure_personal_space() instead of the former ensure_personal_general(), and update_memory, list_memories, and search_memories all accept an optional topic filter parameter. The migration is additive (Migration A); a separate backfill migration (Migration B) drops the legacy personal-only RPCs and RLS policies once all users have been lazily migrated.