fix: Support both Logic-Node condition serialization formats

Logic-Nodes were timing out because UI saves condition as:
  {operands: [...], operator: "and"}

But Backend expected:
  {expression: {operands: [...], operator: "and"}}

This caused node.condition.expression to be None, triggering:
- Logic-Node failures
- Join-Node wait_all timeout
- 504 Gateway Timeout

Fix: Accept both formats by checking for operator/operands attributes
directly on condition, falling back to condition.expression.

Fixes: 504 Gateway Timeout in Training-Tiefenanalyse workflow

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/workflow_executor.py

@@ -398,8 +398,26 @@ def execute_logic_node(
     started_at = datetime.utcnow().isoformat()
 
     try:
-        if not node.condition or not node.condition.expression:
+        if not node.condition:
-            error_msg = f"Logic node {node.id} has no condition/expression defined"
+            error_msg = f"Logic node {node.id} has no condition defined"
+            logger.error(error_msg)
+            return NodeExecutionState(
+                node_id=node.id,
+                status=NodeStatus.FAILED,
+                error=error_msg,
+                started_at=started_at,
+                completed_at=datetime.utcnow().isoformat()
+            )
+
+        # Handle both serialization formats:
+        # UI format: condition = {operands: [...], operator: "and"}
+        # Expected format: condition = {expression: {operands: [...], operator: "and"}}
+        if hasattr(node.condition, 'operator') and hasattr(node.condition, 'operands'):
+            expression = node.condition  # UI format (direct)
+        elif hasattr(node.condition, 'expression'):
+            expression = node.condition.expression  # Expected format (wrapped)
+        else:
+            error_msg = f"Logic node {node.id} has invalid condition structure (missing operator/operands)"
             logger.error(error_msg)
             return NodeExecutionState(
                 node_id=node.id,
@@ -410,7 +428,7 @@ def execute_logic_node(
             )
 
         # 1. Evaluiere Bedingung
-        result, error = evaluate_logic_expression(node.condition.expression, context)
+        result, error = evaluate_logic_expression(expression, context)
 
         if error:
             # Fehler bei Evaluation → Fallback anwenden