SQL Server, how to execute a job against all databases on server?
Create a SQL Agent Job with a Job Step of type "Powershell" on the SQL Server instance where your databases reside.

# Define the query to be run against each database
Set-Variable -Name MyQuery -Value "SELECT @@SERVERNAME"

# Access SQL Server information by using a drive and file system analogy
Set-Location "SQLSERVER:\SQL\localhost\DEFAULT\Databases"

# Get the list of databases
Get-ChildItem | Set-Variable -Name DBList

# Iterate through the list, execute your query
ForEach ( $DB in $DBList) {
    Invoke-Sqlcmd -query $MyQuery -database $DB.Name -ServerInstance 'localhost' -ErrorAction Stop
}


Schedule it like any other job.