Fix regex escaping

Author: tristanlabelle

app/backend/src/couchers/notifications/render_email.py

@@ -154,7 +154,7 @@ def _get_generic_templated_email(user_name: str, notification: Notification) ->
                     else:
                         group_chat_title = None
 
-                    if match := re.search("^You missed (\d+) message(s)", data.message or ""):
+                    if match := re.search(r"^You missed (\d+) message(s)", data.message or ""):
                         missed_count = int(match[1])
                     else:
                         missed_count = 1