Apply cache on dimensions api

Use zip_path and mtime for hashing
It used for weak validation

src/routes/api.cr

@@ -649,21 +649,31 @@ struct APIRouter
         "height" => Int32,
       }],
     }
+    Koa.response 304
     get "/api/dimensions/:tid/:eid" do |env|
       begin
         tid = env.params.url["tid"]
         eid = env.params.url["eid"]
+        prev_e_tag = env.request.headers["If-None-Match"]?
 
         title = Library.default.get_title tid
         raise "Title ID `#{tid}` not found" if title.nil?
         entry = title.get_entry eid
         raise "Entry ID `#{eid}` of `#{title.title}` not found" if entry.nil?
 
-        sizes = entry.page_dimensions
-        send_json env, {
-          "success"    => true,
-          "dimensions" => sizes,
-        }.to_json
+        file_hash = Digest::SHA1.hexdigest (entry.zip_path + entry.mtime.to_s)
+        e_tag = "W/#{file_hash}"
+        if e_tag == prev_e_tag
+          env.response.status_code = 304
+          ""
+        else
+          sizes = entry.page_dimensions
+          env.response.headers["ETag"] = e_tag
+          send_json env, {
+            "success"    => true,
+            "dimensions" => sizes,
+          }.to_json
+        end
       rescue e
         Logger.error e
         send_json env, {