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/lib/services/import/transient-retry.ts

@@ -0,0 +1,25 @@
+const TRANSIENT_ERROR_PATTERN =
+	/fetch failed|network|socket|timeout|timed out|ECONNRESET|ETIMEDOUT|ENOTFOUND|EAI_AGAIN|ECONNREFUSED|EHOSTUNREACH|HTTP 5\d\d|Empty or truncated response|service unavailable|temporarily unavailable/i;
+const sleep = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
+
+/** True for download/connection hiccups worth a retry; false for data errors. */
+export function isTransientImportError(message: string | undefined): boolean {
+	if (!message) return false;
+	return TRANSIENT_ERROR_PATTERN.test(message);
+}
+
+/**
+ * Runs `attempt` again (with backoff) when it returns a transient failure.
+ * Non-transient failures and hard throws are returned/raised immediately.
+ */
+export async function withTransientImportRetry<
+	T extends { ok: boolean; error?: string },
+>(attempt: () => Promise<T>, retries = 2): Promise<T> {
+	for (let n = 0; ; n++) {
+		const result = await attempt();
+		if (result.ok || !isTransientImportError(result.error) || n >= retries)
+			return result;
+		const delay = Math.min(400 * 2 ** n, 2000);
+		await sleep(delay);
+	}
+}