fix(storage): classify backend-init failures too

Ed proposed the obvious end-to-end test — point an S3 entry at
127.0.0.1 with nothing listening, get a refused connection, expect a
transient error — and it would have failed, because `initialize()` was
the one SDK call still wrapped as a plain `internal_error`.

That is the FIRST call both jobs make, so it is what a wrong-endpoint
test actually hits: `backend_consistency` and `backend_migration` each
return `Failed` on init, and every classification added in the previous
commits sits downstream of a path the test never reaches.

Now `head_bucket` goes through `s3_domain_error` like the rest, and both
call sites route through `RunOutcome::from_domain_error`. A refused
connection or a 5xx pauses and can be resumed once the endpoint returns;
a wrong bucket or bad credentials is 4xx and stays terminal, which is
the distinction that makes pausing safe to offer at all.

No cursor at init — nothing has been scanned — so the pause resumes from
the start, which is correct rather than lossy.

Worth noting for `backend_migration`: target init runs BEFORE
`migration_readonly` is engaged, so pausing there holds no write freeze.
An operator can leave it paused indefinitely and resume when the target
comes back, with no read-only window.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/infrastructure/services/backend_migration_service.rs

@@ -452,9 +452,11 @@ impl RecoverableJobHandler for BackendMigrationService {
         // for the swap-hot-swap call in `finish_completed`.
         let target = build_entry_backend_typed(target_entry, &self.storage_path_fallback);
         if let Err(e) = target.initialize().await {
-            return RunOutcome::Failed {
-                message: format!("target backend init: {e}"),
-            };
+            // Runs BEFORE `migration_readonly` is engaged, so pausing
+            // here holds no write freeze — an operator can leave it
+            // paused indefinitely and resume when the target comes back.
+            // A wrong bucket or bad credentials still fails terminally.
+            return RunOutcome::from_domain_error(None, "target backend init", &e);
         }
 
         // All guards passed. Engage server-wide read-only mode for