feat(recipes): skip rate-limited batch-import items and register download 429s (#1085)

Phase 2 of docs/plans/issue-1085-rate-limit-design.md:

- Batch import: items that fail due to vendor rate limiting are now
  SKIPPED with a "re-run the import later" hint instead of FAILED, so a
  transient 429 no longer pollutes failure accounting; the progress
  broadcast carries a rate_limited flag.
- Batch import UI: show a one-time "rate limited — slowing down" toast
  and swap the running status text while rate_limited; i18n keys synced
  to all locales.
- Downloader: download_file / download_to_memory / get_response_headers
  register 429 cooldowns with the RateLimitCoordinator, so subsequent
  API calls queue behind a download-triggered rate-limit window.

locales/ja.json

@@ -1033,6 +1033,8 @@
             "start": "Start Import",
             "startImport": "Start Import",
             "importing": "Importing...",
+            "rateLimitedSlowdown": "[TODO: Translate] Rate limited — slowing down...",
+            "rateLimitedHint": "[TODO: Translate] Some items were skipped due to metadata provider rate limits. Re-run the import later to retry them.",
             "progress": "Progress",
             "total": "Total",
             "success": "Success",
@@ -2038,6 +2040,7 @@
             "batchImportCancelFailed": "Failed to cancel batch import: {message}",
             "batchImportNoUrls": "Please enter at least one URL or file path",
             "batchImportNoDirectory": "Please enter a directory path",
+            "batchImportRateLimited": "[TODO: Translate] Metadata provider rate limit reached — requests are being slowed and some items may be skipped. You can re-run the import later.",
             "batchImportBrowseFailed": "Failed to browse directory: {message}",
             "batchImportDirectorySelected": "Directory selected: {path}",
             "noRecipesSelected": "レシピが選択されていません",