[infra] Storage review: RabbitMQ PVC size, emptyDir sizeLimit, backup annotations

[henzigo]

Context

• **** RabbitMQ PVC default is 1 Gi — a lagging consumer fills it with a few tens of
  thousands of messages, then RabbitMQ starts rejecting publishes.
• **** the webserver source-codes emptyDir has no sizeLimit — a runaway write can
  fill the node disk.
• **** no backup annotations for the RabbitMQ volume.
• **** Redis being intentionally ephemeral is undocumented in the chart.

Proposal

• raise the rabbitmq.persistence.size default to 10Gi + document that PVC expansion
  support depends on the storage class (nfs-client usually allows it) and that existing
  PVCs are NOT resized by helm upgrade (volumeClaimTemplates are immutable — manual PVC edit)
• add sizeLimit to the source-codes emptyDir (values-driven, default e.g. 1Gi)
• optional rabbitmq.persistence.backupAnnotations passthrough (e.g. Velero
  backup.velero.io/backup-volumes: rabbitmq-data)
• comment in redis values/templates documenting the intentional ephemerality
  (pairs with the [security] Move sensitive values out of ConfigMaps and pod specs into Secrets #31 decision)

Tasks

• PVC size default + docs on expansion
• emptyDir sizeLimit (values key on the webserver component)
• Backup annotations passthrough
• Redis ephemerality note
• Golden snapshots, deviations entry (PVC default change affects only NEW installs)