Add Meeting system subject and notification system

- Auto-create a locked "Meeting" subject for every user on registration
  and seed it for all existing users on startup
- Meeting subject cannot be deleted or renamed (is_system flag)
- 5-minute corner toast warning on Dashboard and TV with live countdown,
  dismiss button, and 1-minute re-notify if dismissed
- At start time: full-screen TV overlay with 30-second auto-dismiss,
  automatic pause of running block, switch to Meeting block, and
  auto-start of Meeting timer
- Web Audio API chimes: rising on warnings, falling at meeting start
- Update README with Meeting subject and notification system docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/subjects.py

@@ -101,6 +101,8 @@ async def delete_subject(
     subject = result.scalar_one_or_none()
     if not subject:
         raise HTTPException(status_code=404, detail="Subject not found")
+    if subject.is_system:
+        raise HTTPException(status_code=403, detail="System subjects cannot be deleted")
     await db.delete(subject)
     await db.commit()