Fix PHP 8.2 warnings, add PHPStan, refactor type hinting, and fix notification link and content

[rafaucau]

• Fixed PHP 8.2 warnings by properly handling null parameters.
• Updated minishlink/web-push to the latest version. PHP 8.0 is now minimum supported version.
• Added PHPStan to the project, which will help us find issues more efficiently.
• Refactored the code to use native PHP variable types to make it easier to catch issues.
• Fixed the discussion link and content.

Specifically, I found that the following conditional block was causing problems:

if (in_array($blueprint->getType(), ['newPost', 'byobuPrivateDiscussionReplied'])) {
    $subject = $blueprint->post;
    $subjectModel = CommentPost::class;
}

This block was modifying the $subject and $subjectModel variables unnecessarily. The correct implementation was already present before this block:

$subject = $blueprint->getSubject();
$subjectModel = $blueprint->getSubjectModel();

So, I've removed the problematic conditional block entirely, which resolved the issue.
I have tested the push notifications with new posts and with fof/byobu. Everything is working as expected, with no errors or issues found.

[rafaucau]

Some PHPStan issues to fix:

 ------ ------------------------------------------------------------------------------------------- 
  Line   src/Api/Controller/AddPushSubscriptionController.php                                       
 ------ ------------------------------------------------------------------------------------------- 
  69     Call to an undefined method Flarum\User\User::pushSubscriptions().                         
  91     Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$user_id.           
  92     Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$endpoint.          
  93     Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$expires_at.        
  94     Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$vapid_public_key.  
  95     Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$keys.              
 ------ ------------------------------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------- 
  Line   src/Api/Controller/ResetVAPIDKeysController.php                  
 ------ ----------------------------------------------------------------- 
  47     Dead catch - RuntimeException is never thrown in the try block.  
 ------ ----------------------------------------------------------------- 

 ------ -----------------------------------------------------------
  Line   src/Api/Serializer/PushSubscriptionSerializer.php
 ------ -----------------------------------------------------------
  31     Cannot access property $endpoint on array|object.
  32     Cannot access property $vapid_public_key on array|object.
  33     Cannot access property $expires_at on array|object.
 ------ -----------------------------------------------------------

 ------ ------------------------------------------------------------------------------------
  Line   src/PushSender.php
 ------ ------------------------------------------------------------------------------------
  144    Access to an undefined property Askvortsov\FlarumPWA\PushSubscription::$last_used.
  163    Access to an undefined property Flarum\Database\AbstractModel::$display_name.
  170    Access to an undefined property Flarum\Database\AbstractModel::$type.
  170    Call to an undefined method Flarum\Database\AbstractModel::formatContent().
  172    Access to an undefined property Flarum\Database\AbstractModel::$discussion_id.
  172    Access to an undefined property Flarum\Database\AbstractModel::$number.
  183    Call to an undefined method Illuminate\Contracts\Filesystem\Filesystem::url().
  188    If condition is always true.
  191    Call to an undefined method Illuminate\Contracts\Filesystem\Filesystem::url().
 ------ ------------------------------------------------------------------------------------

[askvortsov1]

Specifically, I found that the following conditional block was causing problems:

Yeah, this made some sense before my most recent refactor, but now it's just a bug. Thanks for this and all the fixes/changes!

I'm actually going to do this as 2 releases: one to clean up bugs and upgrading to web push v7, and another dropping PHP 7 support.