[Feature] Support for JSON gRPC API via Connect-Go

[SuperCoolPencil]

To provide a more robust, typed, and efficient API for external tools and integrations, we should implement a gRPC API.

Using Connect-Go (https://connectrpc.com/) is recommended as it allows:

1. Native gRPC support.
2. gRPC-Web support (useful for browser-based UIs).
3. First-class JSON support over HTTP/1.1 or HTTP/2 (replaces the need for manual http.ServeMux handling for some routes).

This would involve:

• Defining a .proto file that mirrors the DownloadService interface.
• Generating Go code for the service.
• Implementing the service handlers.
• Integrating the Connect handler into the existing HTTP server.