fix(plex): plain text decodes only well-formed character references

html.unescape also decodes the legacy forms without a semicolon, so prose like
"AT&notice" came out as "AT¬ice". The show and collection sync paths are
now tested too.

backend/app/plex/plaintext.py

@@ -17,6 +17,10 @@ _BLOCK = re.compile(r"</?(?:p|div|li|ul|ol|h[1-6]|blockquote)\b[^>]*>", re.IGNOR
 _TAG = re.compile(r"</?[a-zA-Z][^>]*>")
 _BLANKS = re.compile(r"[ \t]*\n[ \t]*\n\s*")
 
+# A well-formed character reference only. `html.unescape` alone also decodes the legacy forms that
+# have no semicolon, so prose like "AT&notice" or "R&copy" came out as "AT¬ice" and "R©".
+_REF = re.compile(r"&(?:#[0-9]+|#[xX][0-9a-fA-F]+|[A-Za-z][A-Za-z0-9]*);")
+
 
 def plain_text(value: str | None) -> str | None:
     """`value` with its markup gone: line breaks kept as newlines, paragraphs as a blank line,
@@ -27,6 +31,6 @@ def plain_text(value: str | None) -> str | None:
         value = _BREAK.sub("\n", value)
         value = _BLOCK.sub("\n\n", value)
         value = _TAG.sub("", value)
-        value = html.unescape(value)
+        value = _REF.sub(lambda m: html.unescape(m.group()), value)
         value = _BLANKS.sub("\n\n", value)
     return value.strip() or None