Sanitizer: a newsletter's <title> showed as a line above the body

<title> is not an allowed tag, so nh3 unwrapped it and kept its text —
"mailer", "Email template" or the subject rendered before the content.
It is now dropped with its content, both in the sanitized HTML and in the
text derived from HTML-only mail (snippet and search). SANITIZER_VERSION
goes to 4 so stored bodies re-sanitize from the raw blob on next render.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/indexer/test_sanitize.py

@@ -17,6 +17,19 @@ def test_scripts_and_handlers_are_removed() -> None:
     assert "hi" in result.html
 
 
+def test_document_title_is_not_rendered() -> None:
+    # Newsletters put "mailer" or the subject in <title>; unwrapping the tag would
+    # show it as a stray line above the body. Hidden preheaders keep their style.
+    html = (
+        "<html><head><title>mailer</title></head><body>"
+        '<span style="display:none">preview</span><p>Hi</p></body></html>'
+    )
+    result = sanitize_html(html, MID)
+    assert "mailer" not in result.html
+    assert '<span style="display:none">preview</span>' in result.html
+    assert "<p>Hi</p>" in result.html
+
+
 def test_remote_image_is_blocked_and_flagged() -> None:
     html = '<p>see <img src="http://tracker.example/pixel.gif"> this</p>'
     result = sanitize_html(html, MID)