0370 — Topic administration moves to an admin object holding the datastore

Context. topic.Exists/Register/Destroy each took (ctx, ds, name), repeating the datastore on every call — package-level functions with no home for shared config or connection state.

Decision. An admin object constructed once with the datastore, so each call needs only (ctx, name) and its own arguments. This is the shape that landed as admin.MessageAdmin (admin.NewMessageAdmin), carrying the topic verbs — RegisterTopic, GetTopic, ListTopics, AlterTopic, RenameTopic, DestroyTopic — plus migration and health reads.

Consequences. One construction site owns the datastore and retry/logger config for all administrative calls; the package-level (ctx, ds, name) functions stop being the public shape.