Asset uploads from network-restricted clients

AI agents running in sandboxed environments (such as Claude Code with limited egress) can now upload files to Vello without needing to reach the Supabase Storage CDN directly. POST the raw file bytes as multipart/form-data to /api/upload on the MCP server URL; the endpoint writes the bytes to Storage server-side and returns a storage_path. Pass that path to attach_asset in mode 2 to register the asset and link it to a thought or insight. The existing inline base64 path (mode 3, ≤ 5 MB) is unchanged; the new endpoint handles larger files up to the 50 MB hard cap.