DB migration: 6-type memory taxonomy + type_origin

memsprout now supports six memory types — Decision, How-To, Standard, Fact, Event, and Note — replacing the original four (Decision, Asset, Fact, Context). A new type_origin field tracks whether a type was set by AI during capture or chosen by you, mirroring the same provenance model used for topic assignment. Existing memories previously classified as Context or Asset have been migrated to Note automatically.