Move 3 Strikes from Admin to Dashboard, add strikes feature

- Adds strikes (0-3) to Child model with migration
- New PATCH /api/children/{id}/strikes endpoint with WebSocket broadcast
- TV dashboard shows red ✕ marks next to child name when strikes > 0
- 3 Strikes card on Dashboard page (removed from Admin)

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

backend/app/models/child.py

@@ -1,5 +1,5 @@
 from datetime import date
-from sqlalchemy import String, Boolean, ForeignKey, Date
+from sqlalchemy import String, Boolean, ForeignKey, Date, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from app.models.base import Base, TimestampMixin
 
@@ -13,6 +13,7 @@ class Child(TimestampMixin, Base):
     birth_date: Mapped[date | None] = mapped_column(Date, nullable=True)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     color: Mapped[str] = mapped_column(String(7), default="#4F46E5")  # hex color for UI
+    strikes: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
 
     user: Mapped["User"] = relationship("User", back_populates="children")  # noqa: F821
     daily_sessions: Mapped[list["DailySession"]] = relationship(  # noqa: F821