Fix duplicate AP posts + backfill posts on new follow

- Stable object IDs per source (no more duplicates)
- Dedup Update activities (Ghost double-webhook)
- Backfill: deliver recent Create activities to new follower inbox

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/actors/routes.py

@@ -89,6 +89,45 @@ async def _send_accept(
         log.exception("Failed to send Accept to %s", follower_inbox)
 
 
+async def _backfill_follower(
+    actor: ActorProfile,
+    follower_inbox: str,
+) -> None:
+    """Deliver recent Create activities to a new follower's inbox."""
+    from shared.events.handlers.ap_delivery_handler import (
+        _build_activity_json, _deliver_to_inbox,
+    )
+    from shared.models.federation import APActivity
+
+    domain = _domain()
+
+    # Fetch recent Create activities (most recent first, limit 20)
+    activities = (
+        await g.s.execute(
+            select(APActivity).where(
+                APActivity.actor_profile_id == actor.id,
+                APActivity.is_local == True,  # noqa: E712
+                APActivity.activity_type == "Create",
+            )
+            .order_by(APActivity.published.desc())
+            .limit(20)
+        )
+    ).scalars().all()
+
+    if not activities:
+        return
+
+    log.info(
+        "Backfilling %d posts to %s for @%s",
+        len(activities), follower_inbox, actor.preferred_username,
+    )
+
+    async with httpx.AsyncClient() as client:
+        for activity in reversed(activities):  # oldest first
+            activity_json = _build_activity_json(activity, actor, domain)
+            await _deliver_to_inbox(client, follower_inbox, activity_json, actor, domain)
+
+
 def register(url_prefix="/users"):
     bp = Blueprint("actors", __name__, url_prefix=url_prefix)
 
@@ -315,6 +354,9 @@ def register(url_prefix="/users"):
         # Send Accept
         await _send_accept(actor_row, body, follower_inbox)
 
+        # Backfill: deliver recent posts to new follower's inbox
+        await _backfill_follower(actor_row, follower_inbox)
+
     async def _handle_undo(
         actor_row: ActorProfile,
         body: dict,