Fixed dangling tags on MySQL after post removal

src/Upgrades/Upgrade39.php

@@ -0,0 +1,22 @@
+<?php
+namespace Szurubooru\Upgrades;
+use Szurubooru\DatabaseConnection;
+
+class Upgrade39 implements IUpgrade
+{
+    public function run(DatabaseConnection $databaseConnection)
+    {
+        $pdo = $databaseConnection->getPDO();
+
+        $pdo->exec('DROP TRIGGER IF EXISTS postsDelete');
+
+        // this triger is needed due to cascade triggers not working in MySQL.
+        // (specifically, postTagsDelete won't get triggered.)
+        $pdo->exec('
+            CREATE TRIGGER postsDelete AFTER DELETE ON posts
+            FOR EACH ROW
+            BEGIN
+                UPDATE tags SET usages = (SELECT COUNT(1) FROM postTags WHERE tagId = tags.id);
+            END');
+    }
+}

src/di.php

@@ -55,6 +55,7 @@ return [
             $container->get(\Szurubooru\Upgrades\Upgrade36::class),
             $container->get(\Szurubooru\Upgrades\Upgrade37::class),
             $container->get(\Szurubooru\Upgrades\Upgrade38::class),
+            $container->get(\Szurubooru\Upgrades\Upgrade39::class),
         ];
     }),