feat: add OpenAPI spec generation with utoipa and justfile

- Add utoipa v5 dependency with ToSchema derives on all REST API DTOs
- Annotate free-function handlers with #[utoipa::path] (trash, share, favorites, recent)
- Create ApiDoc struct with OpenApi derive registering 37 schemas across 7 tags
- Add generate-openapi binary outputting resources/gen/openapi.json
- Serve OpenAPI spec at GET /api/openapi.json (public, no auth)
- Add justfile with common dev commands (build, test, lint, check, openapi, db)

src/application/dtos/folder_listing_dto.rs

@@ -1,11 +1,12 @@
 use serde::Serialize;
+use utoipa::ToSchema;
 
 use super::file_dto::FileDto;
 use super::folder_dto::FolderDto;
 
 /// Combined DTO that returns both sub-folders and files for a given folder
 /// in a single response, eliminating the double-fetch on every navigation.
-#[derive(Debug, Serialize)]
+#[derive(Debug, Serialize, ToSchema)]
 pub struct FolderListingDto {
     /// Sub-folders inside the requested folder
     pub folders: Vec<FolderDto>,