fix: allow empty CRAWL_LIMIT env value

tests/test_config.py

@@ -0,0 +1,13 @@
+from app.config import Settings
+
+
+def test_empty_crawl_limit_is_treated_as_none():
+    settings = Settings(crawl_limit="")
+
+    assert settings.crawl_limit is None
+
+
+def test_numeric_crawl_limit_is_parsed():
+    settings = Settings(crawl_limit="25")
+
+    assert settings.crawl_limit == 25