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_mime.py

@@ -75,6 +75,13 @@ def test_html_only_derives_text() -> None:
     assert "<b>" not in parsed.text_body  # tags stripped for indexing text
 
 
+def test_html_only_text_skips_document_title() -> None:
+    parsed = parse_message(
+        _build(text=None, html="<html><head><title>mailer</title></head><p>Hello</p></html>")
+    )
+    assert parsed.snippet == "Hello"
+
+
 def test_multipart_alternative_prefers_html_for_display() -> None:
     parsed = parse_message(_build(text="text version", html="<p>html version</p>"))
     assert parsed.html_body is not None