fix: _derive_title crashes on non-string content instead of returning Untitled (#1751)

tests/test_derive_title_nonstring.py

@@ -0,0 +1,13 @@
+from routes.document_helpers import _derive_title
+
+
+def test_derive_title_handles_non_string_content():
+    # content normally comes from a document text column, but the helper is
+    # public and a non-string (None / int) made content.strip() raise
+    # AttributeError instead of falling back to a default title.
+    assert _derive_title(None) == "Untitled"
+    assert _derive_title(123) == "Untitled"
+
+
+def test_derive_title_still_reads_markdown_heading():
+    assert _derive_title("# Heading Title\nbody text") == "Heading Title"