feat(l1): add accounts.enabled_l1_categories with default allowlist

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_account_l1_categories_column.py

@@ -0,0 +1,7 @@
+from app.models.account import Account
+
+
+def test_account_has_enabled_l1_categories_default():
+    a = Account(name="Acme", display_code="ABC12345")
+    # Column default is applied at flush; attribute may be None pre-flush.
+    assert hasattr(a, "enabled_l1_categories")