feat(pl): persist create_date on plan create APIs

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_pl_plan_create_date.py

@@ -0,0 +1,11 @@
+from datetime import date
+
+from app.api.pl.plan import _resolve_create_date
+
+
+def test_resolve_create_date_uses_explicit():
+    assert _resolve_create_date(date(2026, 1, 15)) == date(2026, 1, 15)
+
+
+def test_resolve_create_date_defaults_today():
+    assert _resolve_create_date(None) == date.today()