[Feature] Add interval trigger type for workflow schedules

## Feature request

Add an interval-based trigger type for workflow schedules.

## Motivation

Some workflows should run at a fixed interval instead of using a Cron expression. When executions overlap, the scheduler should continue calculating fire times from the configured interval. Users should also be able to configure a finite execution count or unlimited repetition.

## Proposed behavior

- Support Cron and interval trigger types.
- Configure interval by hours, minutes, and seconds.
- Support a repeat count, where `-1` means unlimited repetitions.
- Preview the next interval-based execution times.
- Respect the existing missed-fire policy.