fix: pre-release improvements — scale, correctness, API clarity

#1  count_accepted_followers / get_accepted_followers_page: new DB-side
    methods on FollowRepository — no more loading all followers into memory
    to count or page them.

#2  Move re-follows are now non-blocking: tokio::spawn instead of
    awaiting each sign_and_send inside receive() — inbox handler no longer
    stalls for slow remote servers during account migration.

#3  Remove get_local_objects_for_user from ApContentReader (dead code).
    Backfill and outbox both use the paginated get_local_objects_page.

#6  Rename backfill_outbox → import_remote_outbox with a clear doc
    explaining the direction (import FROM a remote server, not to a follower).

#7  also_known_as: Option<String> → Vec<String> on ApUser, LookedUpActor,
    and DbActor. from_json now stores all aliases; move_act.rs checks all.

Mentions: broadcast_create_note / broadcast_update_note now accept
    mentioned_inboxes: Vec<Url> — delivery goes to followers + mentioned
    actors who aren't already followers. Deduplication is done before
    sending. Pass vec![] if note has no external mentions.

Docs: ApObjectHandler and ApContentReader now have complete doc comments
    with contracts, idempotency guidance, and error-handling semantics.

src/repository/follow.rs

@@ -43,6 +43,16 @@ pub trait FollowRepository: Send + Sync {
     /// followers, excluding blocked actors/domains. DB-side filtering.
     async fn get_accepted_follower_inboxes(&self, local_user_id: uuid::Uuid)
     -> Result<Vec<String>>;
+    /// Count of accepted followers only. More efficient than loading all followers
+    /// and filtering in application memory.
+    async fn count_accepted_followers(&self, local_user_id: uuid::Uuid) -> Result<usize>;
+    /// Accepted followers page for display purposes. `offset` is 0-based.
+    async fn get_accepted_followers_page(
+        &self,
+        local_user_id: uuid::Uuid,
+        offset: u32,
+        limit: usize,
+    ) -> Result<Vec<RemoteActor>>;
 
     // ── Outbound following ──────────────────────────────────────────────────
     async fn add_following(