[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/redis/modules/redis/main.tf

@@ -366,10 +366,10 @@ resource "kubernetes_cron_job_v1" "redis-backup" {
 
                 TIMESTAMP=$(date +%Y%m%d-%H%M)
                 # Trigger a fresh RDB save on the master
-                redis-cli -h redis.redis BGSAVE
+                redis-cli -h redis-master.redis BGSAVE
                 sleep 5
                 # Copy the RDB via redis-cli --rdb
-                redis-cli -h redis.redis --rdb /backup/redis-$TIMESTAMP.rdb
+                redis-cli -h redis-master.redis --rdb /backup/redis-$TIMESTAMP.rdb
                 # Rotate — 28-day retention
                 find /backup -name 'redis-*.rdb' -type f -mtime +28 -delete