refactor: rename bulk operations from /api/books/ to /api/media-items/

- Rename routes: /api/books/bulk-{delete,update} → /api/media-items/bulk-{delete,update}
- Rename route: /api/books/:uuid/download → /api/media-items/:uuid/download
- Update request fields: book_ids → media_item_ids
- Update response fields: success → deleted/updated
- Update result fields: book_id → media_item_id
- Update collection handler: success → added

This change improves API semantic correctness as the system handles
multiple media types (ebooks, comics, manga), not just books.

BREAKING CHANGE: All bulk operation endpoints and field names renamed

internal/router/media.go

@@ -48,7 +48,7 @@ func registerMediaRoutes(cfg *Config) {
 	admin.DELETE("/media-items/:id", cfg.MediaHandler.DeleteMediaItem)
 
 	// Download route (public)
-	e.GET("/api/books/:uuid/download", cfg.MediaHandler.DownloadBook)
+	e.GET("/api/media-items/:uuid/download", cfg.MediaHandler.DownloadBook)
 
 	// Shelf management (protected)
 	protected.POST("/devices/:id/shelves", cfg.MediaHandler.AddToShelf)
@@ -56,8 +56,8 @@ func registerMediaRoutes(cfg *Config) {
 	protected.DELETE("/devices/:id/shelves", cfg.MediaHandler.RemoveFromShelf)
 	protected.DELETE("/devices/:id/shelves/clear", cfg.MediaHandler.ClearShelf)
 
-	// Bulk book operations (protected)
-	books := protected.Group("/books")
-	books.POST("/bulk-delete", cfg.MediaHandler.HandleBulkDelete)
-	books.POST("/bulk-update", cfg.MediaHandler.HandleBulkUpdate)
+	// Bulk media operations (protected)
+	mediaItems := protected.Group("/media-items")
+	mediaItems.POST("/bulk-delete", cfg.MediaHandler.HandleBulkDelete)
+	mediaItems.POST("/bulk-update", cfg.MediaHandler.HandleBulkUpdate)
 }