docs(sync): scope normalize_description to channels + lock line-ending behavior

Review follow-up. The docstring said "channel/video description" but the helper is
wired only to the channel store sites; clarify it is deliberately channel-only
(video descriptions are user-facing prose where paragraph breaks carry meaning,
unlike a channel's metadata block) so it doesn't read as an asymmetric fix. Also
document that str.splitlines() folds CR/CRLF and Unicode line separators to "\n",
and lock it with a test.

backend/tests/test_utils.py

@@ -50,3 +50,8 @@ def test_normalize_description_all_blank_collapses_to_empty():
 
 def test_normalize_description_single_line_unchanged():
     assert normalize_description("just one line") == "just one line"
+
+
+def test_normalize_description_folds_crlf_line_endings():
+    # splitlines() folds CR/CRLF to \n — a documented, benign line-ending normalization.
+    assert normalize_description("a\r\nb\r\n\r\nc") == "a\nb\nc"