Add four tests that verify the tags-as-identity ownership transition
correctly persists to the database when converting a user-owned node
to a tagged node via SetTags:
- TestSetTags_ClearsUserIDInDatabase: verifies user_id is NULL in DB
- TestSetTags_NodeDisappearsFromUserListing: verifies ListNodes by user
- TestSetTags_NodeStoreAndDBConsistency: verifies in-memory and DB agree
- TestSetTags_UserDeletionDoesNotCascadeToTaggedNode: verifies user
deletion does not cascade-delete tagged nodes
Three of these tests currently fail because GORM's struct-based
Updates() silently skips nil pointer fields, so user_id is never
written as NULL to the database after SetNodeTags clears it in memory.
Updates #3161
580dcad683