media assets: add image redirect routes.

Add URLs that redirect from a media asset to the image:

* https://danbooru.donmai.us/media_assets/6961761/original -> https://cdn.donmai.us/original/2c/4f/2c4ff193b0fc7106e59b2f6696a68a02.jpg
* https://danbooru.donmai.us/media_assets/6961761/sample   -> https://cdn.donmai.us/sample/2c/4f/sample-2c4ff193b0fc7106e59b2f6696a68a02.jpg
* https://danbooru.donmai.us/media_assets/6961761/720x720  -> https://cdn.donmai.us/180x180/2c/4f/2c4ff193b0fc7106e59b2f6696a68a02.webp
* https://danbooru.donmai.us/media_assets/6961761/360x360  -> https://cdn.donmai.us/180x180/2c/4f/2c4ff193b0fc7106e59b2f6696a68a02.jpg
* https://danbooru.donmai.us/media_assets/6961761/180x180  -> https://cdn.donmai.us/180x180/2c/4f/2c4ff193b0fc7106e59b2f6696a68a02.jpg

This is useful if you have a media asset ID and want to get to the image
without looking up the image URL in the API.

This endpoint is rate-limited to 5 requests per second. It's not meant
to be used for things like thumbnail galleries or bulk scraping images.

app/controllers/media_assets_controller.rb

@@ -3,6 +3,8 @@
 class MediaAssetsController < ApplicationController
   respond_to :html, :json, :xml
 
+  rate_limit :image, rate: 5.0/1.seconds, burst: 50
+
   def index
     @limit = params.fetch(:limit, CurrentUser.user.per_page).to_i.clamp(0, PostSets::Post::MAX_PER_PAGE)
     @preview_size = params[:size].presence || cookies[:post_preview_size].presence || MediaAssetGalleryComponent::DEFAULT_SIZE
@@ -33,4 +35,12 @@ class MediaAssetsController < ApplicationController
       respond_with(@media_asset)
     end
   end
+
+  def image
+    media_asset = authorize MediaAsset.find(params[:media_asset_id])
+    variant = media_asset.variant(params[:variant])
+    raise ActiveRecord::RecordNotFound if variant.nil?
+
+    redirect_to variant.file_url
+  end
 end

app/policies/media_asset_policy.rb

@@ -5,6 +5,10 @@ class MediaAssetPolicy < ApplicationPolicy
     true
   end
 
+  def image?
+    can_see_image?
+  end
+
   def can_see_image?
     record.post.blank? || record.post.visible?(user)
   end

config/routes.rb

@@ -146,7 +146,9 @@ Rails.application.routes.draw do
       get :check, to: redirect {|path_params, req| "/iqdb_queries?#{req.query_string}"}
     end
   end
-  resources :media_assets, only: [:index, :show]
+  resources :media_assets, only: [:index, :show] do
+    get "/:variant", to: "media_assets#image", as: :image
+  end
   resources :media_metadata, only: [:index]
 
   resources :ai_tags, only: [:index]