Add break time feature to schedule blocks

- Admin: per-block "Break Time" checkbox + duration (min) setting; new
  Break Activities section (global list, same pattern as Morning Routine)
- Dashboard: break timer section appears on blocks with break enabled;
  Start/Pause/Resume/Reset controls work independently of the main timer
- TV: left column switches to amber break badge + countdown during break;
  center column shows configurable Break Activities list
- Backend: break_time_enabled/break_time_minutes columns on schedule_blocks
  (auto-migrated on startup); break_activity_items table + CRUD router;
  break timer events (break_start/pause/resume/reset) stored as TimerEvents
  and broadcast via WebSocket; break_activities included in dashboard
  snapshot and session_update broadcast

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

backend/app/models/schedule.py

@@ -43,6 +43,8 @@ class ScheduleBlock(Base):
     label: Mapped[str | None] = mapped_column(String(100), nullable=True)  # override subject name
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)
     order_index: Mapped[int] = mapped_column(Integer, default=0)
+    break_time_enabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+    break_time_minutes: Mapped[int | None] = mapped_column(Integer, nullable=True)
 
     template: Mapped["ScheduleTemplate"] = relationship("ScheduleTemplate", back_populates="blocks")
     subject: Mapped["Subject | None"] = relationship("Subject", back_populates="schedule_blocks")  # noqa: F821