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/app/indexer/mime.py

@@ -83,7 +83,14 @@ def _addresses(msg: EmailMessage, header: str) -> list[dict[str, str]]:
 
 def _html_to_text(html: str) -> str:
     """Derive plain text from HTML by stripping all tags (SPEC §6.4 nh3 strip)."""
-    return nh3.clean(html, tags=set(), attributes={}, strip_comments=True)
+    return nh3.clean(
+        html,
+        tags=set(),
+        attributes={},
+        strip_comments=True,
+        # The document <title> is not body text; unwrapped it would lead the snippet.
+        clean_content_tags={"script", "style", "title"},
+    )
 
 
 def _get_body_content(msg: EmailMessage, subtype: str) -> str | None: