Fix DAG visualization and step link handling

- Handle dict inputs ({"node": "id"}) when building DAG edges
- Add normalize_inputs() to convert dict inputs to node IDs for steps
- Fix _parse_inputs to use _json.loads (correct import alias)
- Add SOURCE/EFFECT/SEQUENCE colors to node color maps

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database.py

@@ -1159,11 +1159,11 @@ def _parse_inputs(inputs_value):
         return inputs_value
     if isinstance(inputs_value, str):
         try:
-            parsed = json.loads(inputs_value)
+            parsed = _json.loads(inputs_value)
             if isinstance(parsed, list):
                 return parsed
             return []
-        except (json.JSONDecodeError, TypeError):
+        except (_json.JSONDecodeError, TypeError):
             return []
     return []