Improve cache size check to be more performant

Summing the sizes of each cached file every time is very inefficient.
Instead we can simply store the cache size in an constant and increase
it everytime a file is added into the cache.

spec/http_server/handlers/static_assets_handler_spec.cr

@@ -198,5 +198,36 @@ Spectator.describe StaticAssetsHandler do
     end
   end
 
+  it "Will not cache additional files if the cache limit is reached" do
+    5.times do |times|
+      data = "a" * 1_000_000
+
+      make_temporary_file("test cache size limit #{times}") do |temporary_file, file_link|
+        cycle_temporary_file_contents(temporary_file, data) do
+          response = handle HTTP::Request.new("GET", file_link)
+          expect(response.status_code).to eq(200)
+          expect(response.body).to eq(data)
+        end
+
+        response = handle HTTP::Request.new("GET", file_link)
+        expect(response.status_code).to eq(200)
+        expect(response.body).to eq(data)
+      end
+    end
+
+    # Cache should be 5 mb so no more files will be cached.
+    make_temporary_file("test cache size limit uncached") do |temporary_file, file_link|
+      cycle_temporary_file_contents(temporary_file, "a") do
+        response = handle HTTP::Request.new("GET", file_link)
+        expect(response.status_code).to eq(200)
+        expect(response.body).to eq("a")
+      end
+
+      response = handle HTTP::Request.new("GET", file_link)
+      expect(response.status_code).to eq(200)
+      expect(response.body).to_not eq("a")
+    end
+  end
+
   after_each { Invidious::HttpServer::StaticAssetsHandler.clear_cache }
 end