feat(import): durable batch checkpoint and transient auto-retry

Mirror interactive batch runs into the import-job store so interrupted imports (restart, time-out, disconnect) can be resumed from Import History. Items are checkpointed as they settle (coalesced, serialized saves) and the mirror starts 'running' so the boot-time worker marks it 'interrupted' instead of double-importing; done items are never re-imported. Add bounded backoff retry for transient download/connection failures before marking an item failed, and point the client's time-out/network toasts at Import History.

src/components/admin/studio/studio-client.tsx

@@ -933,10 +933,12 @@ export function StudioClient({
 			if ((err as Error).name === "AbortError") {
 				toast.info("Import cancelled");
 			} else if ((err as Error).name === "TimeoutError") {
-				toast.error("Import timed out. Retry or reduce concurrency.");
+				toast.error(
+					"Import timed out. Progress was saved — open Import History to resume failed items.",
+				);
 			} else if (err instanceof TypeError && err.message?.includes("fetch")) {
 				toast.error(
-					"Network error during upload. Check your connection and retry.",
+					"Network error during upload. Progress was saved — open Import History to resume failed items.",
 				);
 			} else {
 				toast.error(