feat: presentation defaults + scheduler active-only

Add Settings → Events (Presentations) defaults (interval, page-progress,
auto-progress) persisted via /api/system-settings
Seed defaults in init_defaults.py (10/true/true)
Add Event.page_progress and Event.auto_progress (Alembic applied)
CustomEventModal applies defaults on create and saves fields
Scheduler publishes only currently active events per group, clears retained
topics when none, normalizes times to UTC; include flags in payloads
Docs: update README, copilot instructions, and DEV-CHANGELOG
If you can split the commit, even better

feat(dashboard): add presentation defaults UI
feat(api): seed presentation defaults in init_defaults.py
feat(model): add Event.page_progress and Event.auto_progress
feat(scheduler): publish only active events; clear retained topics; UTC
docs: update README and copilot-instructions
chore: update DEV-CHANGELOG

scheduler/db_utils.py

@@ -189,7 +189,9 @@ def format_event_with_media(event):
                 "type": "slideshow",
                 "files": [],
                 "slide_interval": event.slideshow_interval or 5000,
-                "auto_advance": True
+                "auto_advance": True,
+                "page_progress": getattr(event, "page_progress", True),
+                "auto_progress": getattr(event, "auto_progress", True)
             }
 
             # Avoid per-call media-type debug to reduce log noise