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/sanitize.py

@@ -17,7 +17,7 @@ import nh3
 
 # Bump when the sanitization policy changes; body_parts.sanitizer_v records it so a
 # render re-sanitizes stale content from the raw blob (SPEC §10.4).
-SANITIZER_VERSION = 3
+SANITIZER_VERSION = 4
 
 _ALLOWED_TAGS: set[str] = {
     # structure
@@ -218,8 +218,10 @@ def sanitize_html(
         link_rel="noopener noreferrer nofollow",
         strip_comments=True,
         # nh3 strips <script>/<style> *content* by default; we allow <style> (above)
-        # and sanitize its CSS ourselves, so only script content is stripped here.
-        clean_content_tags={"script"},
+        # and sanitize its CSS ourselves, so its content stays. <title> is dropped
+        # with its text: as a disallowed tag nh3 would unwrap it, and the document
+        # title ("mailer", the subject) would render as a line above the body.
+        clean_content_tags={"script", "title"},
         attribute_filter=attribute_filter,
     )
     cleaned = _sanitize_style_elements(cleaned, allow_remote_images, state)