Topics: General topic replaces Unfiled, with spaces backfill and topic management UI

Spaces now automatically include a General topic that catches miscellaneous memories — replacing the Unfiled bucket with a real, named topic. All existing space memories have been auto-filed into General (with a backfill endpoint available to re-run the LLM classifier for better topic placement). The General topic cannot be edited or deleted, appears first in the topic filter strip, and the cap of 20 topics still applies to user-created topics only — General doesn't count. Topic management is accessible directly from the space view via the Topics & Settings link.