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/app/utils.py

@@ -39,11 +39,14 @@ def to_tsquery_str(q: str) -> str | None:
 
 
 def normalize_description(text: str | None) -> str | None:
-    """Strip whitespace-only lines from a channel/video description before storing. YouTube
-    descriptions pad contact/link blocks with runs of blank lines; keeping them wastes storage,
+    """Strip whitespace-only lines from a CHANNEL description before storing. YouTube channel
+    descriptions pad their contact/link blocks with runs of blank lines; keeping them wastes storage,
     bloats the About popover, and adds nothing to the search document. Drops purely-empty lines and
-    right-strips each kept line. None stays None; all-blank collapses to "". Applied at the
-    sync/store site so the DB (and the derived search_vector) holds the clean text."""
+    right-strips each kept line; `str.splitlines()` also folds CR / CRLF and Unicode line separators
+    to "\\n" (a benign line-ending normalization). None stays None; all-blank collapses to "".
+
+    Deliberately CHANNEL-only — video descriptions are left with their original formatting: they are
+    user-facing prose where paragraph breaks carry meaning, unlike a channel's metadata block."""
     if text is None:
         return None
     kept = [line.rstrip() for line in text.splitlines() if line.strip()]

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"