feat: add endpoint to add files to envelope

Add new POST endpoint /api/v1/file/{uuid}/add-file that allows
adding multiple PDF files to an existing envelope in DRAFT status.

- Validates envelope exists and is in DRAFT status
- Accepts multipart/form-data with 'files' field
- Supports multiple file uploads
- Returns list of added files with metadata
- Sets parentFileId to associate files with envelope

Signed-off-by: Vitor Mattos <[email protected]>

Author: vitormattos