Segments and bumpers for Cloud Playout schedules

Cloud Playout schedules now support a full segment-and-bumper model for ad insertion and branded breaks.

• Add multiple segments per program with start/end controls and a 30-minute upper bound enforced in the UI.
• Pre-roll and post-roll bumpers can be assigned per program with persistent IDs so the same bumper can be reused across schedules.
• Editing a program preserves existing segments and bumpers; the UI disables overlapping changes that would create invalid timelines.
• Segment endpoints are centralized so the same flow powers schedule channels and live event playouts.
• Order changes and re-ordering are restricted when a program is live to avoid disturbing an active stream.

Notifications for create/update/delete are routed through the centralized Sonner stack with channel-aware copy.