* 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
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