fix(run-queue): prevent concurrency keys from bloating master queue shards (#3219)

Queues with concurrency keys now appear as a single entry in the master
queue instead of one entry per key. This prevents high-CK-count tenants
from consuming the entire `parentQueueLimit` window and starving other
tenants on the same shard.

A new per-queue **CK index** (sorted set) tracks active concurrency key
sub-queues. The master queue gets one `:ck:*` wildcard entry per base
queue. Dequeuing from that entry round-robins across sub-queues,
maintaining per-CK concurrency tracking and fairness.

All existing operations (enqueue, dequeue, ack, nack, DLQ, TTL expiry)
are CK-index-aware and keep the index consistent. Old-format entries
drain naturally during rollout — no migration step needed, single
deploy.

Author: ericallam

.server-changes/ck-index-master-queue-dedup.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Concurrency-keyed queues now use a single master queue entry per base queue instead of one entry per key. Prevents high-CK-count tenants from consuming the entire parentQueueLimit window and starving other tenants on the same shard.