feat: add conditional asset embedding with dev/prod build separation

- Add embed-assets feature flag to make rust-embed/mime_guess optional
- Update Justfile with backend-dev command for development (no embedded assets)
- Add CORS middleware when assets not embedded (for Vite proxy)
- Improve db recipe with Bun script supporting start/reset/rm commands

src/web/mod.rs

@@ -1,5 +1,6 @@
 //! Web API module for the banner application.
 
+#[cfg(feature = "embed-assets")]
 pub mod assets;
 pub mod routes;