refactor(backend): normalize naming convention to backend rather storage

no ambiguity with the backend rather storage

src/application/dtos/settings_dto.rs

@@ -259,7 +259,7 @@ pub struct StorageEntrySummaryDto {
     ///   - See which pairs are configured + their SSH-style
     ///     fingerprints without inspecting `.env`.
     ///   - Cross-reference the head pair against the `head_key_fp`
-    ///     from the last `storage_rotate` completion — if they
+    ///     from the last `backend_rotate` completion — if they
     ///     match AND `failed = 0`, every on-disk blob is under the
     ///     head, and non-head pairs are safe to remove.
     #[serde(default)]

src/application/ports/blob_storage_ports.rs

@@ -136,12 +136,12 @@ pub trait BlobStorageBackend: Send + Sync + 'static {
     /// that need the on-disk BYTES to change even when the CONTENT
     /// hash doesn't:
     ///
-    /// * `storage_rotate` — rewrites every blob under the head pair's
+    /// * `backend_rotate` — rewrites every blob under the head pair's
     ///   format (legacy → v1 header, old key → new key, plaintext ↔
     ///   encrypted). If the target's `put_blob_from_bytes` silently
     ///   skipped, rotation would report success while leaving the old
     ///   format on disk.
-    /// * `storage_migration` — same story when a target already has a
+    /// * `backend_migration` — same story when a target already has a
     ///   blob at that hash from an earlier state (Ed hit this on
     ///   2026-08-01 in the S3 → local migration test).
     ///