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/favorites_dto.rs

@@ -1,5 +1,6 @@
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
+use utoipa::ToSchema;
 
 use super::display_helpers::{
     category_for, format_file_size, icon_class_for, icon_special_class_for,
@@ -7,7 +8,7 @@ use super::display_helpers::{
 
 /// DTO for favorites item, enriched with item metadata via SQL JOIN
 /// so the frontend does not need N+1 requests to resolve names/sizes.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
 pub struct FavoriteItemDto {
     /// Unique identifier for the favorite entry
     pub id: String,
@@ -84,7 +85,7 @@ impl FavoriteItemDto {
 }
 
 /// Result DTO for batch add-to-favorites.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
 pub struct BatchFavoritesResult {
     /// Statistics about the batch operation
     pub stats: BatchFavoritesStats,
@@ -93,7 +94,7 @@ pub struct BatchFavoritesResult {
     pub favorites: Vec<FavoriteItemDto>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
 pub struct BatchFavoritesStats {
     /// How many items were requested
     pub requested: usize,