Add strike events to activity log

Record a StrikeEvent row whenever a strike is added or removed,
and surface them in the activity log timeline with timestamp,
child name, and whether the strike was added or removed.

- New strike_events table (auto-created on startup)
- children router records prev/new strikes on every update
- GET /api/logs/strikes and DELETE /api/logs/strikes/:id endpoints
- Log view merges strike entries into the timeline (red dot,
  "✕ Strike added (2/3)" / "↩ Strike removed (1/3)")

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

backend/app/routers/children.py

@@ -5,6 +5,7 @@ from sqlalchemy import select
 
 from app.dependencies import get_db, get_current_user
 from app.models.child import Child
+from app.models.strike import StrikeEvent
 from app.models.user import User
 from app.schemas.child import ChildCreate, ChildOut, ChildUpdate
 from app.websocket.manager import manager
@@ -89,7 +90,9 @@ async def update_strikes(
     child = result.scalar_one_or_none()
     if not child:
         raise HTTPException(status_code=404, detail="Child not found")
+    prev = child.strikes
     child.strikes = body.strikes
+    db.add(StrikeEvent(child_id=child.id, prev_strikes=prev, new_strikes=body.strikes))
     await db.commit()
     await db.refresh(child)
     await manager.broadcast(child_id, {"event": "strikes_update", "strikes": child.strikes})