Update creative manifest

curl -X PUT https://api.interchange.io/api/v2/buyer/campaigns/cmp_987654321/creatives/cm_abcdef \
  -H "Authorization: Bearer $SCOPE3_API_KEY" \
  -H "Content-Type: multipart/form-data" \
  -F 'metadata={
    "name": "Q2 hero video v2",
    "message": "Tighter 30s cut — stronger CTA",
    "delete_asset_ids": ["asset_001"],
    "new_assets": [
      { "filename": "hero-30s-v2.mp4", "asset_type": "VIDEO", "label": "main_video" }
    ]
  };type=application/json' \
  -F "files=@hero-30s-v2.mp4"

{
  "creative_id": "cm_abcdef",
  "campaign_id": "cmp_987654321",
  "name": "Q2 hero video v2",
  "message": "Tighter 30s cut — stronger CTA",
  "template_id": "video_standard",
  "brand_domain": "acme.com",
  "format_id": { "id": "video_standard", "agent_url": "https://agent.example.com" },
  "preview_url": "https://storage.googleapis.com/creatives/cm_abcdef/preview.html",
  "assets": [
    {
      "asset_id": "asset_002",
      "name": "main_video",
      "original_filename": "hero-30s-v2.mp4",
      "asset_type": "VIDEO",
      "content_type": "video/mp4",
      "file_size": 7903112,
      "public_url": "https://cdn.example.com/creatives/cm_abcdef/hero-30s-v2.mp4",
      "asset_source": "USER_UPLOADED",
      "created_at": "2026-06-07T17:10:00Z"
    }
  ],
  "sync_status": { "synced": true, "agent_count": 3, "last_synced_at": "2026-06-07T17:11:00Z" },
  "created_at": "2026-06-07T15:04:00Z",
  "updated_at": "2026-06-07T17:10:00Z"
}