Batches never split a postcode 🟩

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

backend/app/modelling/batching.py

@@ -16,4 +16,18 @@ def pack_postcode_batches(
     """Pack *properties* into batches of ~*batch_size*, never splitting a
     postcode across batches. A single postcode larger than *batch_size*
     becomes its own oversized batch."""
-    raise NotImplementedError
+    groups: dict[str, list[FilteredProperty]] = {}
+    for prop in properties:
+        groups.setdefault(prop.postcode or "", []).append(prop)
+
+    batches: list[list[FilteredProperty]] = []
+    current: list[FilteredProperty] = []
+    for group in groups.values():
+        if current and len(current) + len(group) > batch_size:
+            batches.append(current)
+            current = list(group)
+        else:
+            current.extend(group)
+    if current:
+        batches.append(current)
+    return batches