fix(phpstan): resolve 6 pre-existing type errors + 2 cs-fixer nits

- AdminAuth: drop dead password_hash() === false check (PHP 8 returns string)
- ExportController: remove redundant !== null after isset() on duration fields
- SystemController: $def['json'] is always present, drop ?? false
- TfeController: simplify always-true type guard when sorting TFE files
- validate-file-fragment: AV 5GB override condition was always true, apply unconditionally
- tests: single-quote string, sort use statements (php-cs-fixer)

tests/phpunit/MarkdownHelperTest.php

@@ -1,8 +1,8 @@
 <?php
 
-use PHPUnit\Framework\TestCase;
 use League\CommonMark\CommonMarkConverter;
 use League\CommonMark\Extension\HeadingPermalink\HeadingPermalinkExtension;
+use PHPUnit\Framework\TestCase;
 
 /**
  * MarkdownHelperTest — TOC extraction from markdown headings.

tests/phpunit/ThesisCreateValidationTest.php

@@ -347,7 +347,7 @@ class ThesisCreateValidationTest extends TestCase
     public function testSynopsisSingleLineFieldTrimmed(): void
     {
         $post = $this->validPost();
-        $post['synopsis'] = "   texte qui flotte  ";
+        $post['synopsis'] = '   texte qui flotte  ';
 
         $data = $this->validate($post);
         $this->assertSame('texte qui flotte', $data['synopsis']);