feat(api): split Redis connection for BullMQ vs cache (RFC-004 Phase 3 WS1)

Introduce getRedisConnection('cache' | 'queue') so ops can point BullMQ at
a separate Redis instance from the cache/throttler/ws-adapter without a
code change. Falls back to REDIS_HOST/PORT/PASSWORD when REDIS_QUEUE_*
vars are unset, so dev and single-instance deploys are unchanged.

- New helper + describeRedisTopology() (safe summary, never leaks password)
- BullModule.forRoot now uses the queue connection
- .env.example documents optional REDIS_QUEUE_HOST/PORT/PASSWORD
- 6 unit tests cover defaults, fallback, precedence, shared/split topology,
  and password leak prevention

Refs: GOO-175

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.env.example

@@ -32,6 +32,19 @@ REDIS_PORT=6379
 REDIS_PASSWORD=CHANGE_ME_IN_PRODUCTION
 REDIS_URL=redis://:${REDIS_PASSWORD}@${REDIS_HOST}:${REDIS_PORT}
 
+# -----------------------------------------------------------------------------
+# Redis — Queue (BullMQ)
+#
+# RFC-004 Phase 3: the async backbone (BullMQ) can point at a Redis instance
+# separate from cache / throttler / websocket to keep hot cache traffic from
+# starving queue operations. If unset, queue traffic falls back to the cache
+# REDIS_* vars above (single-instance dev and small deployments keep working
+# unchanged).
+# -----------------------------------------------------------------------------
+# REDIS_QUEUE_HOST=
+# REDIS_QUEUE_PORT=
+# REDIS_QUEUE_PASSWORD=
+
 # -----------------------------------------------------------------------------
 # Typesense
 # -----------------------------------------------------------------------------