feat: add POLLING_LIMIT_PER_HOST to limit concurrent requests per host

Each batch of feeds sent to the worker pool is now guaranteed to contain unique feed URLs.

When `POLLING_LIMIT_PER_HOST` is set, an additional limit is applied to the number of concurrent requests per hostname, helping to prevent overloading a single server.

Note: Additional requests may still be made during feed refresh. For example, to fetch feed icons or when the web scraper is enabled for a particular feed.

internal/api/category.go

@@ -143,6 +143,7 @@ func (h *handler) refreshCategory(w http.ResponseWriter, r *http.Request) {
 	batchBuilder.WithUserID(userID)
 	batchBuilder.WithCategoryID(categoryID)
 	batchBuilder.WithNextCheckExpired()
+	batchBuilder.WithLimitPerHost(config.Opts.PollingLimitPerHost())
 
 	jobs, err := batchBuilder.FetchJobs()
 	if err != nil {