feat: add safe_render using Jinja2; add tests

tests/test_templates.py

@@ -0,0 +1,19 @@
+import pytest
+
+from app.templates import safe_render
+
+
+def test_safe_render_legacy_and_nested():
+    tpl = "收到{actual_ref_amt}元 by {trans_order_info.remark}"
+    ctx = {"actual_ref_amt": 88.88, "trans_order_info": {"remark": "order123"}}
+    out = safe_render(tpl, ctx)
+    assert "88.88" in out
+    assert "order123" in out
+
+
+def test_safe_render_missing_var_raises():
+    tpl = "Hello {missing_var}"
+    with pytest.raises(Exception):
+        safe_render(tpl, {})
+
+