Add unpublish (Delete) support + improve object IDs

- on_post_unpublished handler sends Delete/Tombstone activity
- Create/Update objects use post URL as id (for Delete reference)
- Delete objects use Tombstone type

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

events/handlers/federation_handlers.py

@@ -163,10 +163,51 @@ async def on_product_listed(event: DomainEvent, session: AsyncSession) -> None:
     )
 
 
+# -- Post unpublished (delete from federation) ---------------------------------
+
+async def on_post_unpublished(event: DomainEvent, session: AsyncSession) -> None:
+    """Send a Delete activity when a post is unpublished."""
+    p = event.payload
+    user_id = p.get("user_id")
+    post_url = p.get("url", "")
+
+    if not services.has("federation") or not user_id:
+        return
+
+    actor = await services.federation.get_actor_by_user_id(session, user_id)
+    if not actor:
+        return
+
+    # Find the original Create activity for this post
+    existing = await services.federation.get_activity_for_source(
+        session, "Post", event.aggregate_id,
+    )
+    if not existing:
+        return  # Never published to federation, nothing to delete
+
+    try:
+        await services.federation.publish_activity(
+            session,
+            actor_user_id=user_id,
+            activity_type="Delete",
+            object_type="Tombstone",
+            object_data={
+                "id": post_url,
+                "formerType": "Article",
+            },
+            source_type="Post",
+            source_id=event.aggregate_id,
+        )
+        log.warning("Published Delete for Post#%d", event.aggregate_id)
+    except Exception:
+        log.exception("Failed to publish Delete for Post#%d", event.aggregate_id)
+
+
 # -- Registration --------------------------------------------------------------
 
 register_handler("post.published", on_post_published)
 register_handler("post.updated", on_post_updated)
+register_handler("post.unpublished", on_post_unpublished)
 register_handler("calendar_entry.created", on_calendar_entry_created)
 register_handler("calendar_entry.updated", on_calendar_entry_updated)
 register_handler("product.listed", on_product_listed)