ConnectionRegistry.upsert already preserved isDefault on conflict but
rewrote created_at from the in-memory model. Re-signing in rebuilds the
connection with DateTime.now() under the same stable id, so the row's
creation time jumped forward on every reauth.
That was cosmetic while created_at only drove list ordering. It is now
behaviour: it picks which connection lends a profile its picture, so
re-adding the originally-first connection could hand the avatar to a
later one. remove() also promotes the oldest remaining row to default
and was reading the same restamped value.
Preserve the existing row's created_at on conflict, reusing the lookup
upsert already performs for isDefault.
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