Feat: implement converterArquivo and converterArquivoCustomizado endpoints with text/plain response

[ricxJr]

• Add POST /namespaces/:nsReq/converterArquivo
• Add POST /namespaces/:nsReq/converterArquivoCustomizado
• Return plain text output (text/plain)
• Centralize logic in converterCore
• Validate body fields and allowed flags
• Prepare VSCode to consume response via response.text()"

Issue: #63