Add Pinwheel Clause support for zone-based encounters in route groups

Allows each sub-zone within a route group to have its own independent
encounter when the Pinwheel Clause rule is enabled (default on), instead
of the entire group sharing a single encounter.

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

backend/src/app/api/encounters.py

@@ -50,15 +50,30 @@ async def create_encounter(
             detail="Cannot create encounter on a parent route. Use a child route instead.",
         )
 
-    # If this route has a parent, check if any sibling already has an encounter
+    # If this route has a parent, check if sibling already has an encounter
     if route.parent_route_id is not None:
-        # Get all sibling route IDs (routes with same parent, including this one)
+        # Get all sibling routes (routes with same parent, including this one)
         siblings_result = await session.execute(
-            select(Route.id).where(Route.parent_route_id == route.parent_route_id)
+            select(Route).where(Route.parent_route_id == route.parent_route_id)
         )
-        sibling_ids = [r for r in siblings_result.scalars().all()]
+        siblings = siblings_result.scalars().all()
 
-        # Check if any sibling already has an encounter in this run
+        # Determine which siblings to check based on pinwheel clause
+        pinwheel_on = run.rules.get("pinwheelClause", True) if run.rules else True
+        any_has_zone = any(s.pinwheel_zone is not None for s in siblings)
+
+        if pinwheel_on and any_has_zone:
+            # Zone-aware: only check siblings in the same zone (null treated as 0)
+            my_zone = route.pinwheel_zone if route.pinwheel_zone is not None else 0
+            sibling_ids = [
+                s.id for s in siblings
+                if (s.pinwheel_zone if s.pinwheel_zone is not None else 0) == my_zone
+            ]
+        else:
+            # No pinwheel clause or no zones defined: all siblings share
+            sibling_ids = [s.id for s in siblings]
+
+        # Check if any relevant sibling already has an encounter in this run
         existing_encounter = await session.execute(
             select(Encounter)
             .where(