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/service/backfill.rs

@@ -8,7 +8,19 @@ use crate::{activities::CreateActivity, actors::get_local_actor, federation::ApF
 use super::{ActivityPubService, delivery::send_with_retry};
 
 impl ActivityPubService {
-    pub async fn backfill_outbox(&self, outbox_url: &str, actor_url: &str) -> anyhow::Result<()> {
+    /// Fetch a remote actor's outbox and import its content into the local instance.
+    ///
+    /// This is for importing a **remote actor's history** — for example, when you want
+    /// to surface an account's past posts after a local user follows them. It fetches
+    /// pages from `outbox_url` and calls `ApObjectHandler::on_create` for each item.
+    ///
+    /// This is distinct from [`ActivityPubService::run_backfill_for_follower`], which
+    /// sends **your** local content to a new follower's inbox.
+    pub async fn import_remote_outbox(
+        &self,
+        outbox_url: &str,
+        actor_url: &str,
+    ) -> anyhow::Result<()> {
         let client = reqwest::Client::builder()
             .timeout(std::time::Duration::from_secs(
                 super::HTTP_FETCH_TIMEOUT_SECS,