Filters that match no properties refuse the run 🟩

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/modelling/router.py

@@ -83,6 +83,16 @@ async def trigger_run(
     properties: list[FilteredProperty] = resolve_filtered_property_ids(
         session, body.portfolio_id, body.filters
     )
+    if not properties:
+        # A task with zero sub_tasks could never roll up to complete; the
+        # app's preview shows the same zero from the same rule (ADR-0056).
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"The filters resolve to no properties in portfolio "
+                f"{body.portfolio_id} — nothing to distribute."
+            ),
+        )
     batches = pack_postcode_batches(properties)
 
     # Pre-create one sub_task per (scenario, batch) message under the