feat(batch-queue): two-level tenant dispatch for fair queue (#3133)

Replace flat master queue index with two-level tenant dispatch to fix
noisy neighbor problem. When a tenant has many queues at capacity, the
scheduler now iterates tenants (Level 1) not queues, then fetches
per-tenant queues (Level 2) only for eligible tenants.

Single-deploy migration: new enqueues write to dispatch indexes only,
consumer drains old master queue alongside new dispatch path until
empty.

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.