fix(api): standardize error response envelope

backend/src/config/swagger.ts

@@ -481,27 +481,20 @@ See [Sandbox Mode Documentation](../docs/SANDBOX_MODE.md) for details.`,
           type: 'object',
           properties: {
             error: {
-              type: 'string',
-              description: 'Error message',
-              example: 'Resource not found',
-            },
-            code: {
-              type: 'string',
-              description: 'Error code',
-              example: 'NOT_FOUND',
-            },
-            message: {
-              type: 'string',
-              nullable: true,
-              description: 'Human-readable detail (present on many error responses)',
-            },
-            details: {
-              type: 'array',
-              nullable: true,
-              description: 'Structured validation issues (zod) when the error is a 400',
-              items: { type: 'object' },
+              type: 'object',
+              required: ['code', 'message'],
+              properties: {
+                code: { type: 'string', example: 'NOT_FOUND' },
+                message: { type: 'string', example: 'Resource not found' },
+                details: {
+                  type: 'array',
+                  description: 'Structured validation issues when applicable',
+                  items: { type: 'object' },
+                },
+              },
             },
           },
+          required: ['error'],
         },
       },
     },