fix(download): retry on Civitai 429 rate limit instead of removing images from metadata

When Civitai returns 429 (Too Many Requests) during example image
downloads, the previous behavior treated all failures identically and
permanently removed the corresponding images from model metadata —
making them impossible to retry.

This commit adds:
- 429 detection + Retry-After header parsing in download_to_memory
- Exponential backoff retry (up to 3 attempts) in
  download_model_images_with_tracking
- Separate tracking of rate-limited vs permanently failed URLs
- rate_limited_models progress tracking persisted to disk
- Rate-limited models are NOT added to failed_models/processed_models
  so they are automatically retried on subsequent download runs
- Force mode clears failed_models when rate-limited images exist

tests/services/test_issue_760_repro.py

@@ -77,7 +77,7 @@ async def test_reprocessing_triggered_when_folder_missing(monkeypatch, tmp_path)
         model_dir = args[3]
         Path(model_dir).mkdir(parents=True, exist_ok=True)
         (Path(model_dir) / "image_0.png").write_text("fixed")
-        return True, False, []
+        return True, False, [], []
         
     monkeypatch.setattr(download_module.ExampleImagesProcessor, "download_model_images_with_tracking", fake_download_model_images)