fix(import): discover LoRA + checkpoint from modelVersionIds when API meta is null

When CivitAI image API returns meta=null and modelVersionIds at root
level, the import flow now:

- Injects modelVersionIds + browsingLevel into a minimal metadata dict
  so the parser can discover LoRAs and checkpoints (both import-from-url
  and analyze-image paths)
- Adds checkpoint dedup + fallback in the parser's modelVersionIds
  handler to avoid duplicate API calls
- Runs EXIF extraction unconditionally in analyze-image path, then
  merges with API metadata (fixes gen params loss)
- Propagates preview_nsfw_level through all three import paths:
  import-from-url, analyze-image (UI Import), and batch-import,
  plus the frontend save flow

py/services/batch_import_service.py

@@ -523,6 +523,10 @@ class BatchImportService:
                 if payload.get("checkpoint"):
                     metadata["checkpoint"] = payload["checkpoint"]
 
+                nsfw = payload.get("preview_nsfw_level")
+                if isinstance(nsfw, int) and nsfw > 0:
+                    metadata["preview_nsfw_level"] = nsfw
+
                 image_bytes = None
                 image_base64 = payload.get("image_base64")