fix(description): render leaked markdown headers instead of showing raw

AI-generated descriptions ("Letra" / "Chorus" / "Translation") landed
in the DB with markdown prefixes that the sanitiser treated as plain
text — so viewers saw literal "## Letra" and "### Chorus" instead of
headings.

HtmlSanitizer::render() now promotes `##` / `###` at the start of a
line into <h2>/<h3> before sanitising, so the existing allowlist
accepts them and the reader sees real headings. Only touches lines
whose entire content is a heading — markdown mid-sentence is left
alone to avoid false positives.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/Support/HtmlSanitizer.php

@@ -77,6 +77,11 @@ class HtmlSanitizer
     /**
      * Produce safe display HTML. Legacy plain-text values (no tags) are
      * escaped and converted to <br>; rich values are run through clean().
+     *
+     * Also detects a handful of common markdown-ish prefixes that leaked
+     * in through AI-generated descriptions ("## Letra", "### Chorus") and
+     * turns them into real <h2>/<h3> tags before sanitising. Without this
+     * step a reader saw literal "## Letra" instead of a heading.
      */
     public static function render(?string $value): string
     {
@@ -85,6 +90,8 @@ class HtmlSanitizer
             return '';
         }
 
+        $value = self::promoteLeakedMarkdown($value);
+
         if (strip_tags($value) === $value) {
             return nl2br(e($value), false);
         }
@@ -92,6 +99,24 @@ class HtmlSanitizer
         return self::clean($value);
     }
 
+    /**
+     * Convert markdown-style heading prefixes at the start of a line into
+     * real HTML tags, so the sanitiser can keep them. Only touches lines
+     * that start with ##, ###, or **…**; everything else is untouched.
+     */
+    private static function promoteLeakedMarkdown(string $value): string
+    {
+        $lines = preg_split("/\r?\n/", $value);
+        foreach ($lines as $i => $line) {
+            if (preg_match('/^\s*###\s+(.+?)\s*$/u', $line, $m)) {
+                $lines[$i] = '<h3>' . e($m[1]) . '</h3>';
+            } elseif (preg_match('/^\s*##\s+(.+?)\s*$/u', $line, $m)) {
+                $lines[$i] = '<h2>' . e($m[1]) . '</h2>';
+            }
+        }
+        return implode("\n", $lines);
+    }
+
     private static function sanitizeChildren(DOMNode $node): void
     {
         foreach (iterator_to_array($node->childNodes) as $child) {