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/schemas/subject.py

@@ -37,6 +37,7 @@ class SubjectOut(BaseModel):
     color: str
     icon: str
     is_active: bool
+    is_system: bool = False
     options: list[SubjectOptionOut] = []
 
     model_config = {"from_attributes": True}