Consistent entry ID type (32-bit compatibility) (#5213)

* Remove FreshRSS_Searchable for better types
The interface was not used, and it was preventing more precise types for the different `searchById()` methods, as they each have different input and output types.

* Consistent entry ID
Entry IDs (which are 64-bit integers) must be processed as string to be compatible with 32-bit platforms

* Fix type

* A few more related types

* PHPStan level 6

* Some more casts needed

* String cast for htmlspecialchars

app/Controllers/feedController.php

@@ -906,7 +906,7 @@ class FreshRSS_feed_Controller extends FreshRSS_ActionController {
 		]);
 
 		//Get parameters.
-		$feed_id = Minz_Request::param('id');
+		$feed_id = (int)(Minz_Request::param('id', 0));
 		$content_selector = trim(Minz_Request::param('selector'));
 
 		if (!$content_selector) {