Import a complete Smart Form structure from a JSON file previously exported.
  This endpoint accepts the exported JSON file as a multipart/form-data upload.
  
  **Import Process:**
  - Uploads and validates the JSON file structure
  - Generates new IDs for all entities
  - Assigns to the specified tenant and cobrand
  - Preserves relationships and ordering
  - Creates audit trail entries
  
  **Note:** This creates a new Smart Form; it does not update existing ones.
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