Enabe/disable dag regardless of schedule

The ability to globally disable the entire dag so that it cannot be triggered from the api/ui/webhooks/schedule.

Example usecase: You have a set of regular deploy dags that need to be disabled during migration/maintenance window and need to prevent users doing accidental trigger.

There can be `enabled: true` config by default, matching current behavior, and the state can be changed using the api and the ui too.

The requested behavior spans DAG configuration plus the API, UI, webhook, and schedule trigger paths; begin by tracing those entry points and how the enabled-by-default behavior is represented. Done means a DAG is enabled by default, can be globally disabled through the API and UI, and cannot be triggered through any listed path while disabled.

Written by the indexing model from the issue text.