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/models/subject.py

@@ -12,6 +12,7 @@ class Subject(TimestampMixin, Base):
     color: Mapped[str] = mapped_column(String(7), default="#10B981")  # hex color
     icon: Mapped[str] = mapped_column(String(10), default="📚")       # emoji
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
+    is_system: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
     user: Mapped["User"] = relationship("User", back_populates="subjects")  # noqa: F821
     schedule_blocks: Mapped[list["ScheduleBlock"]] = relationship(  # noqa: F821