fix(plex): plain text survives a decimal reference too long to parse

int() refuses a decimal run past 4300 digits, so one such reference in a summary raised out of the sync and the text migration. Any number past seven significant digits is beyond U+10FFFF, so it becomes U+FFFD like any out-of-range reference; leading zeros are dropped first. The rails test's comment now says what it guards: both rails agree with the rollup.

backend/tests/test_plex_plaintext.py

@@ -36,6 +36,9 @@ HU_SUMMARY = "<p>A műsorban minden p&aacute;rosnak stoppal kell eljutnia.</p>"
         # ...and only a whole entity name: a legacy one at its start ("&not", "&amp") is not.
         ("AT&notice; and &ampx;", "AT&notice; and &ampx;"),
         ("&#233;t&#xE9; &copy;", "été ©"),
+        # A decimal reference too long for int() to parse is out of range, like &#x110000;.
+        ("a &#" + "9" * 5000 + "; b", "a � b"),
+        ("&#" + "0" * 5000 + "65;", "A"),
         ("Plain summary.", "Plain summary."),
         ("  padded  ", "padded"),
         ("<p></p>", None),