Merge the off-host backup variable documentation

.env.production.example

@@ -306,6 +306,20 @@ API_TOKEN_HMAC_KEY="<openssl rand -hex 32>"
 # BACKUP_S3_SECRET_KEY=""
 # BACKUP_ENCRYPTION_KEY=""
 
+# The two optional members of the set, both with defaults, so leaving them
+# unset does not disable backups.
+#
+# Region sent with the SigV4 signature. Default "auto", which is what
+# Cloudflare R2 expects; AWS S3 and most MinIO deployments want their real
+# region name here.
+# BACKUP_S3_REGION="auto"
+#
+# How long backup objects are meant to live, in days. Default 30, minimum 1.
+# The worker never deletes an object -- its grant is PutObject, GetObject and
+# AbortMultipartUpload -- so this number is what you set the bucket's own
+# lifecycle rule to. See docs/ops/backup-restore.md.
+# BACKUP_RETENTION_DAYS="30"
+
 
 # -----------------------------------------------------------------------------
 # Audit-trail retention -- optional

docs/ops/backup-restore.md

@@ -54,8 +54,10 @@ Cloudflare R2 the **Object Read & Write** token already covers all three.
 
 ## Bucket lifecycle (recommended)
 
-The worker prunes objects older than `BACKUP_RETENTION_DAYS`, but the
-storage provider's lifecycle rule is the canonical safety net:
+The worker never deletes a backup object: its grant covers PutObject,
+GetObject and AbortMultipartUpload only, so a compromised worker cannot
+wipe the history. Expiry is the storage provider's lifecycle rule, set to
+match `BACKUP_RETENTION_DAYS`:
 
 ```
 Filter: "" (all objects)