fix: escape user input in notification content templates

Discussion thread titles (and other user-controlled context fields —
replier_name, author_name, username) were interpolated raw into
notification.content via `str.format(**context)`. That output is
rendered with Django's `|safe` filter in digest_content.html, which
is included by both the email_digest and batched_email body
templates, so a `<style>` block in a thread title survived into
recipient inboxes as executable CSS on email open — enabling
open-tracking, content spoofing, and phishing.

Escape at the source: in `get_notification_content`, wrap every
context value with `django.utils.html.escape` before
`template.format(**context)`, exempting the two structural keys
(`p`, `strong`) that content_templates use as HTML tag names. This
defends every renderer of `notification.content` in one place.

This is the incomplete-patch companion of GHSA-4xv3-5j4x-q8g4
(CVE-2026-42857), which sanitized the post body via
`clean_thread_html_body()` but did not cover the title path.
Fixes GHSA-rv5w-f4r5-h77g.