feat(backup): the overlay picks the credential — BACKUP_CREDENTIALS_SUFFIX

The previous commit created the two halves. This one wires the choice, so dev renders MinIO
credentials and prod renders R2 credentials from the SAME base manifests (R-ENV-02: the
overlay patch is the entire difference between environments; no base manifest is forked).

    cluster-vars  BACKUP_CREDENTIALS_SUFFIX   base ""  |  dev ""  |  prod "-prod"

    velero HelmRelease  credentials.existingSecret: velero-cloud-credentials${BACKUP_CREDENTIALS_SUFFIX}
    3x CNPG Cluster     s3Credentials.*.name:      <db>-backup-credentials${BACKUP_CREDENTIALS_SUFFIX}

ONE SUFFIX, NOT FOUR SECRET-NAME KNOBS. Four independent knobs can be updated three-quarters
of the way, leaving one consumer reading the OTHER environment's key with nothing rendering
wrong — and StrictPostBuildSubstitutions cannot catch that: strict mode fails on an UNDEFINED
variable, never on a defined-but-stale one. With one suffix the four move together or not at
all. The cost is that the rendered Secret name is not greppable from the consumer, so both
kustomizations spell the two names out in full beside the resources.

THE BASE DEFAULT IS "" AND THAT IS LOAD-BEARING. It renders the UNSUFFIXED names — objects
that exist in every render, and that dev is running today. The alternative failure mode is
strictly worse than the shared-Secret state this replaces: velero's pod hangs in
ContainerCreating forever when `existingSecret` names a missing Secret (the volume never
mounts), and a CNPG Cluster whose s3Credentials Secret is absent stops archiving WAL — which
under archive_mode=on is a filling pgdata PVC, not a missing backup. So an unresolvable
default breaks the referencing cluster HARDER than a wrong key does. Both overlays state the
value explicitly anyway, so neither inherits it silently.

Prod's overlay change is a cluster-vars op only. The velero Flux Kustomization patch is
untouched (it stays decryption-only, and velero stays suspended on prod).

VERIFIED — `kubectl kustomize --load-restrictor LoadRestrictionsNone` on clusters/dev and
clusters/prod for the cluster-vars values, then on each of the four affected tiers (platform,
platform-config, platform/velero, apps/wave-0) piped through `flux envsubst --strict` with
that environment's variables:

  strict envsubst              PASSES for all 8 (env x tier) combinations
  dev   velero existingSecret  velero-cloud-credentials
  dev   keycloak/nextcloud/semaphore-db  <db>-backup-credentials         (both key refs each)
  prod  velero existingSecret  velero-cloud-credentials-prod
  prod  keycloak/nextcloud/semaphore-db  <db>-backup-credentials-prod    (both key refs each)
  both  all EIGHT Secret objects present in the rendered output (dev and prod alike)
  no `${...}` survives in any rendered tier except gitea's three DELIBERATE `$${...}` shell
    escapes in apps/wave-0, which are pre-existing and unrelated.

Read alongside the endpoints the same overlays already set, the two environments are now
isolated on credential AND target:
  dev   velero s3://velero (root) + cnpg s3://cnpg      @ http://192.168.1.9:9000  region us-east-1
  prod  velero k3s-prod-backup/velero + barman prefix   @ https://<acct>.r2.cloudflarestorage.com  region auto

Co-Authored-By: Claude Opus 5 <[email protected]>

clusters/prod/kustomization.yaml

@@ -454,4 +454,29 @@ patches:
       - op: replace
         path: /data/CNPG_BACKUP_PATH
         value: "s3://k3s-prod-backup/barman"
+      # ------------------------------------------------------------------------------------------
+      # WHICH CREDENTIAL SIGNS THE REQUESTS TO THAT BUCKET (per-env credential split, 2026-09-07).
+      # ------------------------------------------------------------------------------------------
+      # The knobs above moved the ENDPOINT and the OBJECT LAYOUT per environment. The credential did
+      # not move with them, and it could not: the four backup-credential Secrets are SHARED BASE
+      # manifests, applied to both clusters by the same Flux tiers. Writing the R2 token into them
+      # gave prod a working key by TAKING DEV'S AWAY — dev would have signed MinIO requests with an
+      # R2 key id, and under archive_mode=on that is a filling pgdata PVC, not a missing backup.
+      #
+      # So each credential is now two objects, and this suffix selects prod's:
+      #     velero-cloud-credentials-prod           the R2 token in the AWS INI `cloud` shape
+      #     <db>-backup-credentials-prod            the same token as ACCESS_KEY_ID / ACCESS_SECRET_KEY
+      # for keycloak-db, nextcloud-db and semaphore-db. Both halves ship to both clusters (they are
+      # base resources); ONLY this value decides which one each consumer reads. It is a SUFFIX rather
+      # than four secret-name knobs deliberately — four could be updated three-quarters of the way,
+      # leaving one consumer on dev's MinIO key with nothing rendering wrong, and strict-mode
+      # substitution cannot catch a defined-but-stale variable.
+      #
+      # THE R2 TOKEN IS THE ONE PROVEN FROM INSIDE PROD (chore/r2-credentials 0a6a862): head-bucket on
+      # k3s-prod-backup returns BucketRegion WNAM, and PUT and DELETE both succeed. Rotating it now
+      # touches ONLY the four `-prod` files; dev's MinIO keys are untouched objects. That isolation is
+      # the structural reason for the split, independent of dev's eventual decommission.
+      - op: replace
+        path: /data/BACKUP_CREDENTIALS_SUFFIX
+        value: "-prod"
       # TODO(WS-INFRA, Phase 7): pin prod node placement once the bare-metal role labels exist.