[redis] Migrate live RW consumers off bare redis.redis hostname

Completes the T0 hostname migration. The `redis.redis` service is a
legacy alias that routes to HAProxy via a `null_resource` selector
patch; `redis-master.redis` is the canonical name that has always
routed to HAProxy directly and health-checks master-only.

Changes:
- redis-backup CronJob: redis-cli BGSAVE + --rdb now target
  redis-master.redis. BGSAVE runs on the master (what we want).
- config.tfvars `resume_redis_url`: unused fallback updated for
  grep hygiene; nothing reads it today.
- ytdlp REDIS_URL default: updated for dev-local runs; production
  already sets REDIS_URL via main.tf:283-285 → var.redis_host.
- immich chart_values.tpl REDIS_HOSTNAME: dead Helm template (values
  block commented out in main.tf:524, Immich deploys as raw
  kubernetes_deployment using var.redis_host). Updated to keep the
  file consistent if someone ever revives it.

stacks/immich/chart_values.tpl

@@ -9,7 +9,7 @@ defaultPodOptions:
 
 env:
   # REDIS_HOSTNAME: '{{ printf "%s-redis-master" .Release.Name }}'
-  REDIS_HOSTNAME: "redis.redis.svc.cluster.local"
+  REDIS_HOSTNAME: "redis-master.redis.svc.cluster.local"
   # DB_HOSTNAME: "postgresql.dbaas"
   # DB_USERNAME: "immich"
   # DB_DATABASE_NAME: "immich"