chore: add server changes file for dispatch shard fix

add server changeset file

Author: ericallam

.server-changes/fix-dispatch-shard-tenant-based.md

@@ -0,0 +1,9 @@
+---
+area: webapp
+type: feature
+---
+
+Two-level tenant dispatch architecture for batch queue processing. Replaces the
+single master queue with a two-level index: a dispatch index (tenant → shard)
+and per-tenant queue indexes (tenant → queues). This enables O(1) tenant
+selection and fair scheduling across tenants regardless of queue count. Improves batch queue processing performance.