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

models/models.py

@@ -156,6 +156,8 @@ class Event(Base):
     loop = Column(Boolean, nullable=True)                # NEU
     volume = Column(Float, nullable=True)                # NEU
     slideshow_interval = Column(Integer, nullable=True)  # NEU
+    page_progress = Column(Boolean, nullable=True)       # NEU: Seitenfortschritt (Page-Progress)
+    auto_progress = Column(Boolean, nullable=True)       # NEU: Präsentationsfortschritt (Auto-Progress)
     # Recurrence fields
     recurrence_rule = Column(String(255), nullable=True, index=True)  # iCalendar RRULE string
     recurrence_end = Column(TIMESTAMP(timezone=True), nullable=True, index=True)  # When recurrence ends