This large PR modernizes the backend of the app:
- Python Dependency Management: Poetry → uv
- Object Storage for local dev: MinIO (now unmaintained) → RustFS
- Makefile Target Standardization to align with other LaSuite repos
- Internationalization Removal on backend: we only care about i18n on the frontend
- Backend dependencies upgrade
Images are embedded as base64 data URLs directly in the BlockNote content,
unlike the message composer which uses blob uploads + CID references.
This approach keeps templates and signatures self-contained without
requiring an attachment system.
Furthermore, a email-safe safe html exporter has been created to
serialize blocknote content into html.
A new backend setting MAX_TEMPLATE_IMAGE_SIZE (default 2 MiB) controls
the maximum allowed image size for these composers.
