perf(ap): backfill new followers once per remote instance, not per follower
Backfill fired 20 Create deliveries on every Follow. Mastodon dedupes notes
per-instance, so the 2nd+ follower from the same instance got the same 20 posts
re-sent for nothing (and old posts don't re-enter a new follower's timeline
anyway). Now we skip backfill when another follower already represents that
instance (same shared_inbox), and deliver to the shared inbox when present.
Co-Authored-By: Claude <noreply@…>