Harden S3 DC handling

POSTINSTALL.md

@@ -55,7 +55,7 @@ Supported variables (these map directly to the fields described in the upstream
 - `S3_PREFIX` (optional path prefix)
 - `S3_ARE_LOCAL_BUCKETS` (set to `false` when your provider uses HTTPS “real” domains instead of MinIO-style LAN endpoints)
 - `S3_FORCE_PATH_STYLE` (set to `true` for MinIO, Cloudflare R2, Backblaze, or any host that requires `https://host/bucket/object` URLs)
-- `S3_PRIMARY_DC`, `S3_SECONDARY_DC`, `S3_COLD_DC`, `S3_DERIVED_DC` (advanced: override the canonical data-center identifiers if your Museum database already uses different keys; defaults follow the upstream docs: `b2-eu-cen`, `wasabi-eu-central-2-v3`, `scw-eu-fr-v3`)
+- `S3_PRIMARY_DC`, `S3_SECONDARY_DC`, `S3_COLD_DC`, `S3_DERIVED_DC` (advanced: pick from the canonical data-center identifiers listed in the upstream docs. The names are hard-coded in Museum; leave them at `b2-eu-cen`, `wasabi-eu-central-2-v3`, `scw-eu-fr-v3` unless you know you need one of the legacy aliases such as `scw-eu-fr`.)
 - Optional replication: define **both** `S3_SECONDARY_*` and `S3_COLD_*` (endpoints, keys, secrets, optional prefixes, DC names) to mirror uploads to a second hot bucket and a third cold bucket. Replication is only enabled when all three buckets are configured; otherwise the app stays in single-bucket mode. See [Ente’s object storage guide](https://ente.io/help/self-hosting/administration/object-storage) for sample layouts and discussion of reliability.
 
 You should never edit the generated `/app/data/museum/configurations/local.yaml` directly. If you need to append extra settings (for example, defining `internal.super-admins`), create `/app/data/config/museum.override.yaml` and add only the keys you want to override. Copying the entire sample `s3:` block from the docs into that file will erase the credentials that the package renders from `s3.env` and break replication.